Adds optional flow type checking

[nsfmc]

flow (flowtype.org) is something on its face i should dislike, but the part that won me over is that it's all opt-in. If you want to use it for a file, you just add //@flow to the first line of it and flow will do the right thing, otherwise, it just ignores the file altogether

[nsfmc]

i find flow to be pretty nice, i use a sublime text/sublimelinter plugin for it, but it is pretty reasonably supported in other editors too. the homepage https://flow.org/ makes a pretty good case for why flow is useful (mostly it finds your bugs for your, but unlike java or C won't stop a file from compiling, it just points it out)

[chiaberry]

sounds good to moi