BuildCommand works similar to exec.Command, but rather than returning an exec.Cmd it returns a BuildFunc that can be reused.

DidChange will scan the provided directory looking for any files that have changed after the provided `since` time.Time.

Run will run all pre BuildFuncs, then the RunFunc, and then finally the post BuildFuncs.

RunCommand works similar to exec.Command, but rather than returning an exec.Cmd it returns a RunFunc that can be reused.

Poller is used to poll a directory and its subdirectories for changes, and then will kick off a rebuild of the app when changes are detected.

BuildFunc is a function that performs a build step.

RunFunc is a function that runs an application asynchronously and returns a function to stop the app.