Provide help via POD documentation.

Fix issue tadzik#91.

The idea is to let the help branch to introspect the POD documentation
for a specific command in order to see if some documentation can be found.
If documentation can be found, it is shown. This allows for:
- invoking help on subcommands, for instance
  rakudobrew help nuke
- invoking help without any argument, that as already done
  simply prints the list of commands
- invoking an alien command, that results in printing out
  the list of available commands.

This requires an explicit dependency on Pod::Usage.

The help is provided as POD documentation with sections named 'COMMAND'
and the name of the command as passed to rakudobrew. The nuke command
has been introduced as an example.

bin/rakudobrew

@@ -257,26 +257,62 @@ EOT
         }
     }
 } else {
-    say "Usage:";
-    say "$brew_name current";
-    say "$brew_name list-available";
-    say "$brew_name build " , (join "|", available_backends(), "all"), " [tag|branch|sha-1]", " [--configure-opts=]";
-    say "$brew_name build zef";
-    say "$brew_name triple [rakudo-ver [nqp-ver [moar-ver]]]";
-    say "$brew_name rehash";
-    say "$brew_name switch ", (join "|", available_backends());
-    say "$brew_name nuke ", (join "|", available_backends());
-    say "$brew_name self-upgrade";
-    say "$brew_name test [", (join "|", available_backends(), "all"), "]";
-    say "$brew_name exec <command> [command-args]";
-    say "$brew_name init";
-    say "$brew_name shell [--unset|version]";
-    say "$brew_name local [version]";
-    say "$brew_name global [version]";
-    say "$brew_name version";
-    say "$brew_name versions # or $brew_name list";
-    say "$brew_name which <command>";
-    say "$brew_name whence [--path] <command>";
+    # here we come for the 'help' command or
+    # nothing the program has been capable to
+    # understand
+
+    if ( @ARGV == 1 ){
+	# the user wants help for a specific command
+	# e.g., rakudobrew help list
+	my $command = $ARGV[ 0 ];
+	say "$brew_name help [$command]";
+
+	require Pod::Usage;
+	my $help_text = "";
+	open my $pod_fh, ">", \$help_text;
+
+	Pod::Usage::pod2usage(
+	    -exitval   => "NOEXIT",  # do not terminate this script!
+	    -verbose   => 99,        # 99 = indicate the sections
+	    -sections  => "COMMAND: " . lc( $command ), # e.g.: COMMAND: list
+	    -output    => $pod_fh,   # filehandle reference
+	    -noperldoc => 1          # do not call perldoc for the high verbosity level (99)
+            );
+
+	# some cleanup
+	$help_text =~ s/\A[^\n]+\n//s;
+	$help_text =~ s/^    //gm;
+
+	$help_text = "Cannot find documentation for [$command]!" if ($help_text =~ /\A\s*\Z/);
+	close $pod_fh;
+	say $help_text;
+	exit;
+    }
+    else { 
+	# here the user does not want any specific help
+	# or we cannot understand what he wants, print
+	# a generic usage message
+	say "Usage:";
+	say "$brew_name current";
+	say "$brew_name list-available";
+	say "$brew_name build " , (join "|", available_backends(), "all"), " [tag|branch|sha-1]", " [--configure-opts=]";
+	say "$brew_name build zef";
+	say "$brew_name triple [rakudo-ver [nqp-ver [moar-ver]]]";
+	say "$brew_name rehash";
+	say "$brew_name switch ", (join "|", available_backends());
+	say "$brew_name nuke ", (join "|", available_backends());
+	say "$brew_name self-upgrade";
+	say "$brew_name test [", (join "|", available_backends(), "all"), "]";
+	say "$brew_name exec <command> [command-args]";
+	say "$brew_name init";
+	say "$brew_name shell [--unset|version]";
+	say "$brew_name local [version]";
+	say "$brew_name global [version]";
+	say "$brew_name version";
+	say "$brew_name versions # or $brew_name list";
+	say "$brew_name which <command>";
+	say "$brew_name whence [--path] <command>";
+    }
 }
 
 exit;
@@ -940,3 +976,28 @@ sub get_git_url {
         return "${protocol}://${host}/${user}/${project}.git";
     }
 }
+
+
+__END__
+
+
+=head1 NAME
+
+rakudobrew - Perl 6 environment manager
+
+=head1 SYNOPSIS
+
+rakudobrew command syntax:
+
+    rakuydobrew <command> [options] [arguments]
+
+
+=head1 COMMAND: nuke
+
+Usage: rakudobrew nuke <version>
+
+The C<nuke> command totally destroy the specified Perl 6 version from your repository
+of installed backends.
+
+
+=cut