Configurable commands for checkers

Hi,

For now all checkers are hardcoded in the plugin. Like here:

    let makeprg = 'jshint ' . shellescape(expand("%")) . args

I.e. jshint can be only one and must be accessible with system PATH variable. It's not so easy to use another path/version/wrapper, that more suitable for my environment.

I propose to make this "jshint" more configurable, and give to user ability to change command of any checker if necessary, using one line in vimrc:

    let g:syntastic_javascript_jshint_cmd = '/path/to/jshint --custom-options'

For that we need to adjust first line above to this form:

    let makeprg = SyntasticCheckerCommand('javascript', 'jshint')  . shellescape(expand("%")) . args

I did it already for all Python checkers and for JS partially (as example).
For details please see comment for SyntasticCheckerCommand in the patch.

Thanks

P.S. Seems that patch can also solve problem #423 and maybe some similar.