Alternate annotation syntax & other goodies

This is the first "production release", adding one feature:

• An alternate annotation syntax for keeping annotations & code separate (thanks @phadej!)

...and a few more minor tweaks and fixes:

• A more self-documenting alias for /*:: (thanks @phadej!)
• Defaulting to colored Flow output (thanks @cletusw!)
• Proper exit value proxying from Flow (thanks @cletusw!)
• Correct treatment of directories ending with .js (thanks @pyryk!)
• Other small fixes

Initial release

Features:

• Basic replacement for flow check
• /*:, /*::, /*flow-ignore-begin*/ and /*flow-ignore-end*/ supported
• Lots of things missing, like proxying arguments to and exit value from flow, colours in output, etc