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.
Preserve order in meta lines with dictionary value
Merge pull request #84 from martijnvermaat/dictionary-meta

Correctly write meta lines with dictionary value