Allow specifying multiple source directories and target directories

[felixhao28]

This PR allows specifying multiple source directories and target directories:

syncdir --source <source_path1> --target <target_path1> --source <source_path2> --target <target_path2>

This is useful when:

1. You don't want to spawn multiple watch processes to sync several directories in watch mode.
2. Target directories overlap. For example, merging several source directories into one target directory, while deleting orphans.

In addition to this feature, I fixed some overall code style/bad smell issues and added proper tests for simple cases.

[hoperyy]

Many changes...

I'll check and it will take a while...