Read manifest from GitHub url or the file system

[g-pavlov]

What this PR does / why we need it:

• Makes use of the configured resource handlers used also by the reactor to read a resources. The order is the same as in the reactor - first the FS handlers is applied and if it doesn't accept the URI we move on to the GitHub implementation.
• config.go was renamed to manifest.go to reflect better its purpose.
• Also a problem was found with GitHub resource handlers failing with NPE for invalid paths provided to their Accept method and this was fixed.

To test:
docforge -d /tmp/docforge-docs -f https://github.com/gardener/documentation/blob/master/.docforge/documentation.yaml --github-oauth-token $GITHUB_TOKEN
and
docforge -d /tmp/docforge-docs -f ../documentation/.docforge/documentation.yaml --github-oauth-token $GITHUB_TOKEN
(assuming ../documentation/.docforge/documentation.yaml is valid path to the same manifest on your local file system)
shall yield the same result.

Which issue(s) this PR fixes:
Fixes #71

Release note:

GitHub URLs are now supported as manifest path flag (`-f`/`--manifest`) arguments, in addition to (local) file system file paths. The `--github-oauth-token-map` and `--github-oauth-token` flags arguments are used to apply authentication token to the request when fetching manifest from a GitHub URL.

[g-pavlov]

/ok-to-test