Consider adding an option to prevent parsing lines starting with `#` as build output

[jkz-stripe]

Opening this as a sort of retroactive PR. I'm really used to the code-first approach, sorry about that! Opening this based on the contribution guidelines.

The test suite my team maintains at Stripe has some tests that contain output which looks like a build comment (two fields starting with a #). In my particular case this is probably OK, since we build all tests in a separate step before running them. So any output of that form is guaranteed to come from tests themselves rather than the compiler. What do you think about adding an option like the one I coded up in #176 to bypass build_output parsing?