Mark mocha and nyc as optional dependencies

[Avaq]

A breaking change that makes so users have to manually install mocha and nyc. The reasoning is taken from fluture-js/fluture-template#1 (comment).

[Avaq]

I'm not sure about this one. With the same reasoning, all dependencies could be marked as optional, because all sanctuary scripts are manually replaceable. That would put the user in a worse position, though, as a bunch of boilerplate is being shoved back onto their plate.

[davidchambers]

Aldwin, what do you think about this now that #64 has been merged?

[Avaq]

Happy to close this