Add support for 'beta' builds

* Beta builds will be between nightlies and official releases in terms of
  stability. Tested at least to make sure there are no obvious bugs, but
  haven't been through 1-2 weeks stabilising period.
* Beta builds will upload crash reports, so I can get early warning of any
  bugs that are creeping in on master.
Bump version numbers to 0.21, as we're closer to that than 0.20 now
hash_version.sh takes a parameter to append to the git hash

* Useful for adding -official (for version releases) or -beta