Support multiple -directory flags

[booleangate]

A single -directory flag assumes a specific project structure wherein the app entry point and all of its dependencies are accessible in a single directory tree. However, this is not always the case.

Consider mono-repos with multiple applications. Often, there are directories for each application and shared dependencies are in a different directory tree along side those applications. Additionally, applications may be categorized (for example backend, frontend, infrastructure, etc), which exacerbates the need for more flexibility in defining watchable directories.

Allowing multiple -directory flags maintains the existing functionality and interface while also supporting different project organizations without having to create complicated and harder to maintain -exclude-dir configurations.

[githubnemo]

Sorry for not getting back to you sooner. There's still one unaddressed review comment (watchDirectories vs. watchDirectory). Do you still want to address this? It's OK if you don't, I would then merge this PR and address it myself afterwards.

[booleangate]

I'm a little swamped right now with work so I don't know when I'll be able to make that change.

[sungwoncho]

I am interested in this feature. Anything I can do to help get this merged?