Provide Tagged Releases (for melpa-stable)

[the-kenny]

It would be very useful to have tagged releases of rust-mode. Main benefits are:

a) Known working versions - not every user wants to run a random git revision
b) Visibility in melpa-stable: Instead of using nightly snapshots melpa-stable uses the latest tagged release of a package
c) If tracking rustc-releases, one could always use the version of rust-mode that works for the current compiler

A simple git tag v1.12.0 on a known-good revision is enough and would provide much value.

[MicahChalmer]

Closing as a dupe of #87. Also see #178 - there's a tag for 0.3.0 now.