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.  

This 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. I also updated the README.md to explain this.

This PR is particularly useful for [capturing animated gifs](https://github.com/mroth/lolcommits/pull/108)!