Integrate empower-assert

[twada]

Integrate teppeis/empower-assert to enable general purpose assertion enhancement 💪

Design decision to be made

• traverse AST twice?
• refactor espower to accept empower-assert's visitor?
• refactor espower to expose its visitor?

[twada]

I've created merge-estraverse-visitors module to merge multiple estraverse visitors into one.
I'm going to integrate empower-assert module with espower by using merge-estraverse-visitors module.

refs power-assert-js/espower#29