Consider adding default value for BN instance

[frangio]

https://github.com/ZeppelinSolutions/chai-bn/blob/1900f60a9d5fbba3d44c755e24b5efe2f81d3a1d/chai-bn.js#L1

We could add a default value of BN = require('bn.js'). What do you think? Coupled with #3 it works out nicely I think.

[nventuro]

chai-bignumber does this, but I'm not convinced it's that useful of a feature: I'd either always have the dependency injected, or always require it inside the plugin, not both.

[frangio]

It's definitely useful if it makes the most common scenario simpler to use. Would it be the most common scenario? I don't see why not.