FiledVersionTagger class to allow code to be aware of tito tagged version

hey there!

We are using this to allow codelines to have tito expand the version tagged.

For example, tito.props has:
```
  [globalconfig]
  default_builder = tito.builder.Builder
  default_tagger = iteng.tagger.FiledVersionTagger
  changelog_do_not_remove_cherrypick = 0
  changelog_format = %s (%ae)


  [version]
  variable_name = VERSION
  file = ./lib/iteng/util/version.rb
  template_file = ./rel-eng/templates/version.rb
```

./rel-eng/templates/version.rb looks like:
```
  module Iteng
    module Util
      $version_name = "$version"
    end
  end
```

and the resulting ./lib/iteng/util/version.rb looks like:
```
  module Iteng
    module Util
      VERSION = "0.0.10"
    end
  end
```

after a `tito tag`