Add support for specifying the priority with addrepo subcommand

[nguyen-phillip]

Don't write priority field in repo config if not specified.
Unspecified priorities are always interpreted as the default value.
Marshal priority values with semantic names when appropriate.
Add tests for writeRepoFile.

Looks like:

% googet -root=/tmp addrepo --file temp.repo foo https://googet.server.com/foo     
Wrote repo file /tmp/repos/temp.repo with content:
- name: foo
  url: https://googet.server.com/foo
  useoauth: false

% googet -root=/tmp addrepo --file temp.repo --priority rollback bar https://googet.server.com/bar     
Appended to repo file /tmp/repos/temp.repo with the following content:
- name: bar
  url: https://googet.server.com/bar
  useoauth: false
  priority: rollback

% cat /tmp/repos/temp.repo 
- name: foo
  url: https://googet.server.com/foo
  useoauth: false
- name: bar
  url: https://googet.server.com/bar
  useoauth: false
  priority: rollback