enhancement: Translate frontmatter data

[jbukuts]

Currently frontmatter data is stripped from the markdown into an JSON object before the document is uploaded for translation. It is then added back in at the end of the process

This issue revolves around adding an option to specify values within the frontmatter that need to be translated and then using the token API to do so.

Ideally this translation should occur after a translated document is successfully retrieved. If a failure occurs at this step then the original text can act as a fallback.