watchman-make should support ignoring some patterns

Here I am trying to use `watchman-make` to run tests on file changes:

watchman-make --root feed2exec/ -p '**/*.py' --run tox

My project is called `feed2exec` and I used the `--root` argument to keep changes to `.tox` and egg files from being picked up, otherwise it would constantly rerun because tox would trigger itself...

... except it *still* triggers itself because tox touches the `feed2exec/_version.py` file on build. I can't figure out a way to use watchman in this use case...

I did manage to use watchman this way:

$ watchman watch feed2exec/
{
"version": "4.9.0",
"watch": "/home/anarcat/src/feed2exec/feed2exec",
"watcher": "inotify"
}
$ watchman -j <