The js syntax checker was 4 checkers stuffed into 1 file. Now it is
broken down so that each checker is in its own file inside
syntax_checkers/javascript/ while syntax_checkers/javascript.vim just
determines which file to source.
Change the names of the global js checker config variables so that
they all start with "syntastic_javascript_". This is purely for
namespacing.
Remove some unneeded logic around the config variables too. Not sure how
it got in there, but it was more confusing than helpful without offering
any benefits...
