Feature Request: add option to enable saving testmondata on KeyboardInterrupt

[zettabyte]

Sometimes when building the initial .testmondata for a large project, I may need to stop the process but would like all data for tests run so far to be saved so that they at least don't need to be run again to build .testmondata.

If possible, it's be nice to have a command-line switch to enable this behavior so when I have to CTRL-C the test run and re-start it again later, I won't be starting over from scratch since building the initial .testmondata can be very slow.

[tarpas]

Thanks, for writing it down. If anybody wants to implement it contact, me. I think a change of storage format would need to be done first.