Add planex-pin 'freeze' command to update base spec files to match pins

[euanh]

After developing for a while with a pinned repository, it would be nice if planex could help to update the base spec file and tag a new release. The planex-pin freeze command (or some other name - perhaps planex-pin release is better) should:

• rewrite the base spec file, generating a new version number and resetting the release number
  • the new version number should be provided on the command line
• prompt the user to fill in the changelog - perhaps helping by printing a summarized list of changes since the last tag
• tag the pinned package source repository with the new version number

The user will be responsible for pushing the tag to the upstream package repository.

[euanh]

@simonjbeaumont: What do you think?

[simonjbeaumont]

Sounds good. I've made a start on this on this branch: master...simonjbeaumont:pin-freeze