Removed clear option (non cross platform), added watching directories, a...

I've made major changes to the code in order to track newly created files in a directory. I've base my modification on the 0.4 version, because I like the python code to be cross platform and the --clear option does not work in Windows. 

I've also put a new config mode using a json file (there is a sample that I use for my own game build pipeline), where you can put a directory to watch, and define a list of commands. Those commands can access environment vars and also some pre-built vars that you can use (see the code on CommandExecutor) so you can have easy access to path components of the file changed.

Then there are the FileMatcher class, or mathers, that are only a regex, that if matches, the commands will be run on that file. 

I know that is a big change, and perhaps you will reject this pull request, but I needed this functionality. Since I've based my work on yours, I think it's fair to let you choose if you want it in your repo.