support shorthand for 100% test coverage check

[gr2m]

• Version: v16.9.1
• Platform: Mac OS X 10.15.7 (19H1417)

I really like tap's --100 flag. Could we add something similar to c8? It would turn

c8 --check-coverage --lines 100 --functions 100 --branches 100 --statements 100  node test.js

into

c8 --100 node test.js

[bcoe]

@gr2m I like this idea 👍

[gr2m]

@bcoe I'm not quite sure how the --check-coverage parameter works.

I we want support both

1. c8 check-coverage --100
2. c8 --100 npm test

but I'm not sure how. I tried to use yargs.middleware() but the code does not seem to be executed at all. See my WIP PR at #332