Make SyntaxCheckers_*_GetLocList() dictionary functions.
Pass a reference to the current checker to syntastic#makeprg#build().
Add an optional 'redirect' argument to CreateAndRegisterChecker().
Change the sh checker to use the new dictionary functions.
Add a new registry method getLocListRaw() (needed for the sh checker).
The variable force_highlight_callback is gone.
Highlight functions are now consistently named
SyntaxCheckers_<filetype>_<checker>_GetHighlightRegex(), and they
take precedence over highlighting based on column.