Some features in Locale aren't available for formatting

[Southclaws]

Hi, thanks for a great library!

I'm working on a system that's dealing with both fiat currencies and a couple of cryptocurrencies and I thought I could easily build a Locale object to describe common crypto formats (all decimal places, symbol after the number, etc)

I have this:

var cryptos = map[string]accounting.Locale{
	"USDC": {
		Name:           "USD Coin",
		FractionLength: 11,
		ThouSep:        ",",
		DecSep:         ".",
		SpaceSep:       " ",
		UTFSymbol:      "",
		HTMLSymbol:     "",
		ComSymbol:      "USDC",
		Pre:            false,
	},
}

But then I realised, the actual formatting API doesn't accept some of these parameters, most importantly Pre which I wanted to use to place the symbol after the value, which seems to be the consensus around crypto currency value formatting:

123.435 USDC

For example.

It seems this could be implemented in the formatMoneyString function if Accounting had a Pre field.

Though, it also begs the question, why not just embed Locale into Accounting?

Anyway, would be happy to submit a PR for formatMoneyString!