currency: added symbol support

This adds the Formatter type and

@gri: please consider the API aspect of this.
Also, shall we call the type Formatter or FormatFunc?

Note that there is no localized number formatter yet, so the
currency rendering is a bit impaired.

Change-Id: Iea44ea4725550688a70a705783aa4e9ef31f56d9
Reviewed-on: https://go-review.googlesource.com/15262
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
7 files changed