Add hide_curreny_symbol option

There are cases when the symbol is not really helpful. For instance, in my
case, when using Romanian Leu (RON) currency, formatting with the L symbol
is not correct. The correct form would be 10 lei/Lei/LEI and not 10 L. In most
cases we just use the currency name (RON). I think it's easier to just set this
option to true rather than adding an extra decorator to do the job.