Let the remote tracking branch be configurable

[m007]

This commit allows the user to define a per repository or per branch
specific configuration of which branch git-radar compares the current
branch to. If no branch is configured explicitly "origin/master" will
be used (being backwards compatible with the current behaviour).

A branch specific remote tracking branch may be configured by executing

git config --local branch."$(git rev-parse --abbrev-ref HEAD)".git-radar-tracked-remote origin/my-branch-to-compare-to

A repository specific remote tracking may be configured by executing

git config --local git-radar.tracked-remote origin/my-branch-to-compare-to

A branch specific setting will overwrite a repository specific
configuration (which will overwrite the default, i.e origin/master).