Add support for --version flag
Improve use of matcher
Improve version help
Put version into user agent string

- move version const into separate package
- call it "package_info" because there's some problem with calling it "version"
Make `versionNumber` a variable that is passed rather than imported

So that we don't create an extra package just for storing/sharing a
version number.
Merge pull request #14 from alphagov/version

Have a version, with `--version` and in user agent