major flow compilation speedup

[opensrcken]

I work in a JS codebase with hundreds of files, and it is much, much faster to run flow against a top-level directory than each individual file, which is what gulp-flowtype does. In a fork, I've added support for passing a directory into gulp.src. I would recommend adding support for that in gulp-flowtype.

[charliedowler]

#42 (comment)