feat(changelog script): Have script produce markdown, add settings

This completely revamps the changelog script. It now generates links to
the relevant commits in the format of:

     - [`sha-1 hash`](link on github) subsystem: [commit message](link)

Additionally, the types of commits shown in the changelog and the root
repository for the link may be set at runtime. Example:

    $ TYPES='feat(;Features' ./generate-changelog.sh v0.9.0
    $ REPOROOT="http://github.com/Xe/deis" ./generate-changelog.sh v0.9.0

This is purely cosmetic and to enable this script to be reused more
easily. The script is now documented and sourceable as a library for
further reuse.