Add --onstart similar to --onexit

[bendtherules]

ptw currently has --onexit, but no --onstart - There should be no reason for this asymmetrical design. There is already --beforerun, but it is run before every run of pytest and so, isn't a good candidate for initialising something just once.

It can be useful for starting things like a external notifier, which wants to run for the whole duration of ptw.

Adding it should be trivial.

[joeyespo]

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.