Moved watch onto dynamic file extensions

I am developing a test framework on top of `mocha` which uses `.json` and `.yaml` files as part of its usage. To make `--watch` work with the relevant files, I have expanded `utils.files` to watch all file extensions we have registered.

Additionally, this patches #804.

If you are interested in the framework, it is called [doubleshot][doubleshot].

[doubleshot]: https://github.com/twolfson/doubleshot