Enhancement: Add ability to filter out prerelease, alpha, etc. on github API calls

[dchurch315]

Requesting a new feature 🧑‍💻

I've made a couple of ext-repos using deb-get and one issue that comes up often is that the API call to grab LATEST can also grabs pre-releases of the app. Most of the examples I've seen in the commits to deal with this issue involve making grep snips on the resulting URL call to get only actual releases but this is a fragile method as each github owner can (and will unfortunately) change their filenames periodically. Likewise this approach is not portable between apps.

I propose adding a variable that would allow adding the prerelease boolean FALSE to the github API call - this will leverage github handling this for deb-get. Perhaps it could even be made the default behavior with the variable allowing folks to force to turn on getting prereleases if they so desire.