Adds a `text_file parameter` for issue #3.
Adds a `text_file parameter` for issue #3.

- `text` always overrides `text_file` (backwards compatible)
- If neither are provded: _(NO MSG)_
- If text file is missing: _(NO MSG FILE)_
- If text file is blank: _(EMPTY MSG FILE)_

navicore/teams-notification-resource