Implement functionality of the entr command line utility

[johnae]

Even though I find entr extremely useful, spook is lightweight enough that it could easily have the same functionality in addition to its quite extensive other functionality. So, eg. spook could be used for more one-off tasks like:

find . -type f | spook -k restart_the_server

Or basically anything else that entr is good for. For those smaller tasks a Spookfile is obviously not needed and should probably not even be loaded.

[johnae]

See #37 for progress.

[johnae]

See the actual command line switches - they don't match entrs exactly.

[johnae]

Available as of f418685 and will be released in 0.8.1.