Point out syntastic global variables earlier

Update README -> FAQ to include an explicit question "How can I pass
additional arguments to a checker" and update to the vimdoc "Configuring
specific checkers" to point users to global variables accepted by
'syntastic#makeprg#build' before pointing them to checker source code.
Merge pull request #526 from cheecheeo/master

Point out syntastic global variables earlier
docs: a couple of trivial updates