Suggestion: currency formatting improvement

Hi, the currency format functionnality should be improved to fit all the possible configurations. For example, here are the values returned by `{{@price,-12.95|format}}` for a few different locales, compared with `money_format` results:
<table>
<tr><th>Locale</th><th>Fat-free format</th><th>PHP money_format</th></tr>
<tr><td>en_US</td><td>$-12.95</td><td>-$12.95</td></tr>
<tr><td>fr_FR</td><td>€-12,95</td><td>-12,95 €</td></tr>
<tr><td>ru_RU</td><td>руб-12,95</td><td>-12,95 руб</td></tr>
<tr><td>da_DK</td><td>kr-12,95</td><td>kr -12,95</td></tr>
</table>

I added support to the code, following what is defined in the C99 standard. Test script [here](http://534f.de/TFg2Xj6V).
Optionally we can use `money_format` if available.