better feature for testing `--fork` option (resubmitted)

Adding this as a separate PR. 

It basically improves the feature testing the `--fork` option.  With this change forking writes to an `lolcommits.pid` file (in `configuration.loldir`) with the forking process id (standard practice for this sort of thing).  Lolcommits captures as normal in the forked process and then deletes this pid file when its done.

The test the checks for the presence of this file and waits until it gets cleared before testing if the capture worked.

:books:  quick history lesson, these changes were originally issued in [this PR](https://github.com/mroth/lolcommits/pull/110) that included some other things, and was eventually reverted when we rolled back the 0.5 release (the one with animated gif generating for the mac)

I think its worth adding this back in to improve the test suite, no new gem release is required.