Add ability to pass one or many directories to jscodeshift. Fixes #21

[cpojer]

This allows us to use jscodeshift without find/xargs which should really help with the cli ergonomics. I've been fighting with this all the time and got really annoyed, so glad I finally got around to change the Runner.

This makes use of node-dir and uses Promises. I converted the Runner code to ES6, pretty sure there are some subjective code style choices but I tried to make the style more "functional" and use const where appropriate.

[fkling]

Can you simply increase the timeout in bin/__tests__/jscodeshift-test.js so that this test passes as well?

[pieterv]

Would be nice to use node-glob instead of node-dir, so you can do nice matching like my-project/**/MyPrefix*.js. Any reason why you didn't use it? if not i can do a PR if find the need for it :)

[cpojer]

I didn't think that was important. In my codemods I always do the filtering in code (like search for specific require statements) and I just run them on everything. If you would prefer node-glob, feel free to send a PR.

[pieterv]

Ok thanks @cpojer, ill see if it comes up and PR if needed :)

[fkling]

I think one of the issues is also that your shell will evaluate the path before it's passed to jscodeshift. However, some shells, with the correct setting, actually support ** natively.
I'm not opposed to it, but I'd also say we just see if there is an actual need for this.