Separate attribute for SpaceAfter

[martinpopel]

The last column (MISC) in CoNLL-U may contain SpaceAfter=No.
SpaceAfter is a standardized language-independent attribute (unlike other features stored in MISC).
We should consider providing a special attribute (setter and getter) for it:
$node->set_space_after(1) if !$node->space_after;
It is shorter (more user friendly, I hope) than accessing it through $node->misc.

Options:

1. extract SpaceAfter from MISC when loading CoNLL-U files and save it there in Write::CoNLLU.
2. keep $node->space_after as an alias/shortcut for $node->misc->{SpaceAfter}, so users are not surprised that it is missing there.
   However, $node->misc->{SpaceAfter} has possible values Yes and No (with Yes being the default - it probably should be never stored there explicitly), while $node->space_after should have the usual Perl boolean values (undef and 1).