[FIX] skip .git and few other files / directory during validation

[Remi-Gau]

• fixes #62

Also comes with some refactoring.

• http server is launched outside of the validate_dir function
• validate_dir now works recursively by:
  1. validating files in a folder
  2. listing directories to validate in that folder and running itself on each.
• list of folders / files to validate automatically exclude a specific list of files / folders

[Remi-Gau]

Note that we may get into this weird behavior where validation return "x conforms" even if all files are skipped.

Possible improvement in another PR: run validation on ALL files and only report ALL errors content at the end. As a user I found it annoying to only get errors one after the other when validating large amounts of files.

[Remi-Gau]

FYI: could run it directly on the whole repo of reproschema-library and cobidas_schema.

[djarecka]

thank you, this looks good! Should I merge it or you want to add the exclude parameter?

[Remi-Gau]

let's keep the PR small and add more features in follow up PR, if the current set up has too much friction.