Deliver stable releases if newer than latest prerelease depending on settings

[safinn]

#20 Maybe I have gone in the right direction to what you are expecting. I tried to explain my thinking below:

1. Checks to see if PRE is true (this flag asks the question if pre-releases should be considered as well as stable releases).
2. Finds the latest pre-release.
3. Check to see if the ONLY_PRE environment variable is true (asks the question if only pre-releases should be cached). If true, this pre-releases is cached. END.
4. If ONLY_PRE is false, the latest stable release is found.
5. Both releases are checked to make sure they have assets.
6. If only one of the releases meets the requirements (step 5), this release is chosen to be cached. END.
7. If both releases meet the requirements, the latest (by published_at date) is cached. END.
8. If no release meets the requirements, nothing is cached (the previous successful cache still gets delivered). END.

TODO

• Edit readme.md to include changes
• Clean comments