Fix #876, Correct handling of quotation marks in commands

- remove extra quotes around `cmd /c` arguments
 - prevented && and similar special cases
- don't strip quotes surrounding command-line arguments
 - prevented command paths and arguments containing spaces
- `CommandGrammar` may need additional generalizations
 - e.g. unquoted term can't contain more than one escape sequence
 - but it's probably good enough for now

src/Microsoft.Framework.ApplicationHost/Impl/Syntax/CommandGrammar.cs

@@ -29,7 +29,9 @@ public CommandGrammar(Func<string, string> variable)
 
             var unquotedTerm = Rep1(unquotedPiece.Or(specialPiece)).Str();
 
-            var quotedTerm = Ch('\"').And(Rep(quotedPiece.Or(specialPiece)).Str()).And(Ch('\"')).Left().Down();
+            var quotedTerm = Ch('\"').And(Rep(quotedPiece.Or(specialPiece)).Str()).And(Ch('\"'))
+                .Left().Down()
+                .Build(str => "\"" + str + "\"");
 
             var whitespace = Rep(Ch(' '));
 

src/Microsoft.Framework.PackageManager/Scripts/ScriptExecutor.cs

@@ -52,9 +52,8 @@ public void Execute(
                     if (!string.IsNullOrEmpty(comSpec))
                     {
                         scriptArguments =
-                            new[] { comSpec, "/C", "\"" }
+                            new[] { comSpec, "/C" }
                             .Concat(scriptArguments)
-                            .Concat(new[] { "\"" })
                             .ToArray();
                     }
                 }