A few regexpengine=1 / vim 7.3 fixes

This commit fixes a couple general issues I found with Regexes.

Certain contained groups would break the ending of a regular expression,
causing the style to bleed out into other parts of the file. By adding
keepend we prevent the regex from leaking out.

Also, matchgroup for jsRegexpGroup must be set before start and end
definitions or else you incur bizarre highlighting behaviours. This has
now been fixed.

It does not fix Vim 7.4 and the regexpengine=2 issues, however these
still are necessary fixes for all users on vim 7.3