Add currency option to Money field type

It currently formats using $ which is no good for anyone on a different currency.

While this could be replaced by specifying a different format string, it would be nice to have an explicit currency option that changes the default.

Alternatively, we could implement language support from numeraljs and set / use its global language instead - this is worth looking into.

[JohnCLo]

looks like the new numeral update broke the old functionality, by renaming “language” to “locale”

   if (this.currency) {
		try {
			numeral.locale(this.currency, require('numeral/locales/' + this.currency));
			numeral.locale(this.currency);
		} catch (err) {
			throw new Error('FieldType.Money: options.currency failed to load.');
		}
	}

[adsp16]

Still cant use pounds as a format string, only defaults to $