Support TOML

[cjdoris]

Allow deps to be specified in a juliapkg.toml file in addition to juliapkg.json.

Default to writing to the TOML file unless the JSON file already exists.

[peterjob-oi]

As someone who uses this package, I would prefer to just allow running juliapkg with a Project.toml rather than a juliapkg.toml. I actually assumed that this is what the variable PYTHON_JULIAPKG_PROJECT was going to determine.