Fix error on output due to mismatched byte/str

[fsimonis]

This PR fixes the issue reported in #25 and the explanation justifies a PR.

We either output the generated graph as a string or render it to bytes (png,pdf).
An output stream either accepts bytes or strings, so step 1 was to encode the str to bytes to make a consistent output.

Next issue is that using an argparse.FileType with wb leads to insonsistent file types.
Nothing and - are interpreted as sys.stdout, which accept str (utf-8).
Passing a file results in a buffer that accepts bytes.

This was fixed in newer versions of python.
The workaround I found was to conditionally write to use args.outfile.write for opened files and args.outfile.buffer.write for stdout.
I just realized that there it is possible to directly specify default=sys.stdout.buffer in the parser. This then consistently leads to args.outfile always being a byte-writable buffer.