document quiet/verbose options

[cbm755]

For now, -quiet and -verbose work, but are not documented.

[n,t,summary] = doctest('foo') is now quiet-by-default, a change in behavior from the last version.

Probably they should be documented, unless we want to wait a bit and see what upstream/testify/etc do about this.

By default, I'm going to tag this 0.8.0... but maybe @catch22 feels strongly that it should be 0.7.0?

[catch22]

No strong feelings either way. 😄