Get unit names or even switch to names instead of symbols

[Enchufa2]

I received a question from a user that was parsing some units from some file, using quantities::parse_units, and then trying to get the names of the units from the symbols. This is possible, but a little tricky right now. It would be nice to implement some function to retrieve the unit name from the symbol (and viceversa), or even some option to switch to names when printing units.