[WIP] Multiple root directories and file system provider support

[geofjamg]

• Do you want to request a feature or report a bug?
  Feature

• What is the current behavior?
  In the current version, we only support one root directory and local default file system provider is used.

• What is the expected behavior?
  We could imagine supporting multiple root directories corresponding either to several file system unrelated directories and/or multiple different data source (local file system, FTP).

• What is the motivation / use case for changing the behavior?

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

(if a question doesn't apply, you can delete it)