fixing custom formats by string

This PR fixes the option to pass a second argument as string to select a custom formatter:

```
{{intlNumber NUM "foo"}}
{{intlDate 1390518044403 "bar"}}
```

It also fixes the message format for something like this:

```
the cost {price, number, foo} will be ready by {deadline, date, bar}
```

where `foo` is a number formatter, and `bar` is a data formatter.