Add basic lua hot-loading support with libfswatch

When a file is required, begin watching it. If the file is changed, re-load it.

[libfswatch](https://github.com/emcrisostomo/fswatch/blob/master/README.libfswatch.md) is a cross-platform library to enable this.

In future, it may be desirable to track the actual modules required by each module and reload dependents after a module is changed.