Support registering a custom assertion module

[ColCh]

How to use it? For example, Chai is very nice. Or assert.js

[sindresorhus]

Yes you can. Updated the readme about it: 3deba51

It won't work with planned assertions though. For that, we'll need an AVA method that hooks the assertion methods in the assertion module you supply so that it can do the assert counting. Shouldn't be that hard, but not top priority.

[ColCh]

Thanks. Any tutorial on how to do that? Code sample will be cool.

[sindresorhus]

How to do what? With assert counting is not possible yet as outlined above. Without assert counting is documented in the readme as already mentioned.

[sindresorhus]

Please read the discussion in #49 before commenting.

We should support any assertion library, but have built-in "adapters" for the most popular ones. By "builtin adapter", I mean users can just pass the instance (e.g. chai) and have it just work.