Separate assertion library from testing library

[kierans]

What problem does this feature solve?

Currently @vue/cli-plugin-unit-mocha presupposes that you want to use chai. I personally prefer to use Hamjest, although others may have their own preferences. Can the assertion library be decoupled from using Mocha? Ideally I'd like to see as an option when generating a Vue app the ability to select Mocha and then pick an assertion library.

What does the proposed API look like?

1. Investigate adding options into generator prompts to select assertion framework (ie: chai, hamjest)
2. Update file generation to use options.