Add --onstart similar to --onexit about pytest-watch HOT 1 CLOSED

Unlike the --onexit option, there isn't a strong case for its dual aside from the purity of symmetry.

--onstart can be accomplished outside of ptw pretty easily (such as myscript && ptw). However, --onexit solves a practical problem where you need to run some explicit cleanup code before terminating from an external script environment, e.g. a Makefile or npm run. (In retrospect, --finally may be a better name than --onexit to match the common try-finally concept. This name might better indicate that there isn't some "missing" dual feature.)

Symmetry is a nice property to have in theory and mathematics, but it can easily lead to over-engineering and clutter if there's no other purpose. Adding a feature is usually much easier than maintaining it, so triviality isn't exactly a good indicator of necessity.

Thanks for the suggestion.

from pytest-watch.