Implement 'format' filter

[flexferrum]

Implement 'format' filter according to the jinja2 specification:

Apply python string formatting on an object:

{{ "%s - %s"|format("Hello?", "Foo!") }}
    -> Hello? - Foo!

Actually, {fmt} library has got all necessary facilities for analyze python format strings so, it could be good idea to reuse this facilities during resolving this issue.

[mateka]

I would like to try do it! :)

[flexferrum]

You're welcome!

[mateka]

I have looked into this during weekend, and there are three options:

1. using format internals
2. limiting number of format params
3. creating new format library.
   It is because it is not possible to "unpack" dynamic vector into static variadic args.

[flexferrum]

I supposed to follow option 1. I. e. use format internals. We're able to stick to the particular version of {fmt} (i. e. 6.0) and use it's facilities:

1. Format string parser: https://github.com/fmtlib/fmt/blob/b615eca9641f12edb6713058437fd8e83c03fe09/include/fmt/format.h#L2433
2. Variable number of arguments storage: https://github.com/fmtlib/fmt/blob/e9b9b0aefaf3dacf8554ba93bd404248574945ed/include/fmt/core.h#L1189