Correctly write meta lines with dictionary value

Write meta lines with a dictionary-like value as

    ##meta=<field=value,field=value,...>

instead of as the Python dictionary string representation. This is a
fix for jamescasbon#83 and a generalization of jamescasbon#81. A
regression compared to jamescasbon#81 is that the order of fields in
a `contig` line is no longer defined.

The implementation still feels a bit hackish and there are probably other unknown cases not handled properly, but this should fix a bunch.