Add preprocessor to use Google Translate API translate missing translations

[tony84727]

Draft preprocessor configuration:

// diplomat.yaml
preprocessors:
  - type: google-translate
    options:
      from: en
      to:
        - zh-TW
        - jp

The preprocessor should query API only while the translation is missing.
Get the API key by interactive mode or environment variables.

See Google Translate API doc

[tony84727]

Adding this as a preprocessor is a little awkward. The operation requires accessing Google Translate API. Users are charged base on the word count, and the generated translations should be saved, only get updated when the source(from translation) changed.

[tony84727]

I am going to give it a try. Add a preprocessor for Google Translate, and use another file to cache result.