Add new localized versions of lang.FormatNumber etc.

[bep]

We currently have this in the lang template funcs package:

func NumFmt(precision, number interface{}, options ...interface{}) (string, error) {}

The above isn't connected to any language definition, which makes it a little hard to use in a multilingual context.

With the introduction of time.Format in a57dda8 we now have the additional methods in the interface below (localized for "all" languages in the world).

We need to keep the existing function, so to avoid naming conflict, I suggest we add the below on the form

• FormatNumber
• FormatPercent
• FormatCurrency
• FormatAccounting

That way we could also, if we want, add an alias to FormatTime.

Thoughts? @jmooring @moorereason @regisphilibert and gang.

type interface Translator {
        // returns 'num' with digits/precision of 'v' for locale and handles both Whole and Real numbers based on 'v'
	FmtNumber(num float64, v uint64) string

	// returns 'num' with digits/precision of 'v' for locale and handles both Whole and Real numbers based on 'v'
	// NOTE: 'num' passed into FmtPercent is assumed to be in percent already
	FmtPercent(num float64, v uint64) string

	// returns the currency representation of 'num' with digits/precision of 'v' for locale
	FmtCurrency(num float64, v uint64, currency currency.Type) string

	// returns the currency representation of 'num' with digits/precision of 'v' for locale
	// in accounting notation.
	FmtAccounting(num float64, v uint64, currency currency.Type) string
}