i18n - add mark_translated to highlight extracted strings

I didn't have much luck using Unicode as per Foreman, as table_print got confused and messed up the layout.  Strangely, it was better with its "better" multibyte option disabled, but that's probably not best.
Was there any reason for expanding the function and not using
```ruby
msg = FastGettext::TranslationMultidomain.D_(key)
DL + msg + DR unless msg.nil?
```
Similarly for the other functions, all described here http://ruby-doc.org/gems/docs/f/fast_gettext-0.8.0/FastGettext/TranslationMultidomain.html.