Enhancement: format (u)int8 types correctly to number, if formatted with %d, &i, %u

If I try to format a (u)int8 number, Boost.Format considers this as (unsigned) char, even if I use `%d`, `%i` or `%u`.

[This is a known and rather old issue](http://boost.2283326.n4.nabble.com/format-show-uint8-t-as-a-number-not-a-char-td2563278.html).

The workaround is to cast the number to (u)int32, before formatting.

Nevertheless the current behavior leads to very subtle bugs, because the developer has to check the type of the number to be printed very carefully.

Plain `printf` handles (u)int8 numbers in combination with `%d`, `%i` or `%u` correctly.

Boost version: 1.68 (32bit)
Compiler: MSVC 15.8