Implement VersionChecker#versions_for and #version_of methods

Summary: As a packager, I would like a high-level / single-value
snapshot of package versions in multiple targets & subtargets

Background: Polisher can query multiple targets (git, koji, yum, other)
as well as subtargets (git branches, koji tags, yum repos, etc)
in a single call returning metadata such as package versions for each.

Inorder to facilitate metadata abstraction (which by nature is lossy
but compact) we introduce a few methods that select the versions
most frequent found in the polisher query targets and subtargets

Implementation: Adds to methods to VersionChecker:

- versions_for: returns the most frequently referenced version in
                each configured polisher target (across all subtargets)
- versions_of:  returns the most frequently referenced version across
                all configured polisher targets

Includes specs for updated / new features.