Add support to run over multiple files or directories

[boyter]

Implements request #61 where you can pass in multiple files or directories on the command line and scc will process them as instructed. Biggest change is that the file walker is no longer responsible for closing its stream. This makes the code that spawns them a little more ugly since it now needs to have a waitgroup to track this but I don't seen an obvious way to improve it.

Appears to work over everything I tried. There may be the old ulimit issue again if the directories passed in are large enough, but I was not able to trigger it on my machine.

[boyter]

Ill be away till around the 29th of May so if you are happy merge, if not ill look after any feedback when I get back. No issues either way.

[dbaggerman]

Looks good to me. Having a monitor goroutine close a chan when a waitgroup completes is a pattern I've used for dealing with multiple writers. I usually prefer to break out explicit manager/worker functions rather than using nested inline go func() but it's not a big deal.