add a copy of the jslint checker as a possible checker for html files

ref #801, copy of jslint with s/javascript/html/ as new checker, created by:

    cp syntax_checkers/javascript/jslint.vim syntax_checkers/html/jslint.vim
    sed -i "s/javascript/html/g" !$

This would be useful for anyone that wants to check javascript embedded in html.

Can be enabled in .vimrc using:

    let g:syntastic_html_checkers=['jslint']

Appreciate that it's a kludgey solution, since it dupes a lot of code in the two *jslint.vim*s, so won't be offended if you don't accept the PR.