Make forkable

[jamestalmage]

My attempt at fixing #57.

// all submodules now export a function that takes a `fork` instance
module.exports = function (fork) {

  // this use of fork.use replaces "require" everywhere.
  var types = fork.use(require('../lib/types')); 

  // modules that exported something should now return it from the function instead.
  return exportsObject;
};

Fully customizable definitions can now be achieved:

var types = require('ast-types/fork')([
  require('ast-types/def/core'); // just the core
]);

One potential downside (and I don't see it being a big deal):

var types1 = require('ast-types/fork')([
  require('ast-types/def/core');
]);

var types2 = require('ast-types/fork')([
  require('ast-types/def/core');
]);

// This throws with "[object Object] is not a Def"
// It probably should throw, but the error message needs improvement.
types1.Type.def('Node').isSupertypeOf(types2.Type.def('Expression'))

[benjamn]

Thanks for this @jamestalmage!

If we're going to change all that whitespace anyway, I would really love to finally switch to using two spaces per tab for indentation. Would that be easy to fix?

[jamestalmage]

I would think so. I will update soon.

[hzoo]

@jamestalmage Can you update with what needs to be done here? Looks like we would need this to progress on babel/babel#3561 to account for babel 6 node types

[jamestalmage]

Rebased, and eliminated some irrelevant style-only changes that were cluttering the diff:

Best viewed via https://github.com/benjamn/ast-types/pull/145/files?w=1

@benjamn - I never did change to two tab spaces, but now I am concerned doing so would create headaches for @hzoo rebasing #162 once this is merged.

Would you prefer I hold off on the whitespace changes for now? @hzoo?

[hzoo]

Might make it easier to read but you can always use ?w=1 and in my case i'm only adding a new file so it's fine

[benjamn]

Thanks again @jamestalmage, especially for your patience. 🍴 🎉

[jamestalmage]

🎉 happy this made it in!