Feat: -l | --list flag for install command

**Rationale**

* Maintain the 'versions' file as stable releases of go are released.
* Each version is on different line so adding a version just adds a line in git instead of replacing a line.
* Was quick and simple to implement.

*Might not be a maintainable solution, but seemed ok to me since we're not compiling so rc's and betas don't really make sense to download.*