document quiet/verbose options

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?