Allowing vimrc to override jshint errorformat and exe

I'm using the Rhino jshint package, which means that "jshint" isn't executable. I defined g:syntastic_javascript_jshint_exe as something else in my .vimrc, but jshint.vim was still looking for jshint. I made it so that it now tests to see if that was user-defined.

The Rhino package also spits out a different error format, so I checked for a new varable, g:syntastic_javascript_jshint_errorformat, in case the user needs to specify that as well.