config feature

[Kostov6]

What this PR does / why we need it:
This PR adds the ability to specify the default branches and number of versions per manifest uri in the docforge config file

Example config file:

defaultBranches:
  https://github.com/Kostov6/scala/blob/DEFAULT_BRANCH/foo.yaml: foo
lastNVersions:
  https://github.com/Kostov6/documentation/blob/DEFAULT_BRANCH/.docforge/documentation.yaml: 3
  https://github.com/Kostov6/documentation/blob/DEFAULT_BRANCH/.docforge/documentationFoo.yaml: 4
  https://github.com/Kostov6/scala/blob/DEFAULT_BRANCH/foo.yaml: 3

Right now this is the priority that the specification of these parameters follow(highest to lowest):
Default branch: --main-branches {manifest}={branchName} -> config file -> --main-branches default={branchName} -> master
NVersions: --versions {manifest}={nVersions} -> config file -> --versions default={nVersions} -> 0

Which issue(s) this PR fixes:
Fixes #223

Special notes for your reviewer:
Some changes in the handler files are just code refactoring

Release note:

Default branches and number of versions can now be configured in the docforge config file

[gardener-robot]

@Kostov6 You need rebase this pull request with latest master branch. Please check.

[dimitar-kostadinov]

/lgtm