Adding new LOLCOMMITS_FORK (or --fork) option to fork the runner capturing

Use the LOLCOMMITS_FORK env variable or --fork argument in the lolcommits
command to fork the runner capturing process to a new thread.  Helps when the
capture is slow and you want to return to a usable terminal prompt right away.

Using fork, some plugins may prompt in your terminal after the initial capture.
README also updated to explain this.