Implement automatic release detection

This PR solves the problem of requiring release definition files. Now when you install a release via `exenv install <version>`, it will first check for a matching local custom definition, and if one is not found, then it will query the GitHub API for known releases, if a match is found in the list of releases, it will install via the tarball.

This means that as soon as a new Elixir release is published to GitHub, users of exenv/elixir-build can immediately install that release via `exenv install`. Furthermore, one could easily implement an `exenv install latest` option that takes advantage of the fact that we can pull the latest releases from GitHub.

Questions/comments welcome!