Support for formatting

[MichaelSasser]

Hi,

Jinja has this nice support for formatting like:

{{ '0x%0X' % payload.id }}

It would be nice to have something like this in tera, specially for integers file types. Maybe something more rust like:

0x{{ payload.id | format(":X") }}
or 
0x{{ payload.id | format=":X" }}

[Keats]

I would take a PR for that if it doesn't bring some big dependencies.

[rimutaka]

I need to add decimal separators to numbers, e.g. 1000 -> 1,000 or 123456 -> 123,456, which may be 123.456 in some locales. @Kixiron , would I be able to do that with PR #517?