Add MIME language to UTF8 conversion code

* src/mlang-charset.cpp, src/mlang-charset.h: New.
* src/Makefile.am: Update accordingly.

This is COM based code but as we are a COM addin anyway
we can utilize it to convert MIME strings to unicode and
then from unicode to UTF8.
Our internal "we do everything in UTF-8" has a bit overhead
here but I prefer it.