Store matching pattern in ProcessStarter.pattern_match

[dholth]

• fixes #160

When I was doing this in my ProcessStarter subclass, I had to read lines in reverse to avoid finding previous runs in a persistent log file. May need to check against that here.

Checklist:

• [ ] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [ ] Add or update relevant docs, in the docs folder and in code.
• [ ] Add an entry in CHANGELOG.rst, summarizing the change and linking to the issue.
• [ ] Run pre-commit hooks and fix any issues.
• [ ] Run pytest and make sure no tests failed.