releases: add script to assign short labels to downloadable assets

This script can be used by anyone (with a valid github token) to fix up the label text on the green download buttons in our releases.  See:
    https://github.com/TauLabs/TauLabs/releases/

Normally, the buttons have the full file name on them.  After running this script, the buttons have a shorter name (determined by the file extension) that shows which OS the download is for.

Running it without any options (or without a GITHUB_TOKEN env var) will simply display the valid releases.

Adding the ````-r <release_id>```` option will also list the assets available in that release.  Adding ````-r```` and ````-f```` and a ````GITHUB_TOKEN```` env var will use the github API to fix the label text.