[pm3] accept tags in either .txt or .toml

[shish]

Looking at

• custom .txt format
• json
• yaml
• toml

with the goals of

• be human readable + writable
• be machine readable + writable
• work consistently across multiple implementations (upload, edit, processmedia, lint, scripts)

txt:

• the most compact format (the only format which supports chained tags directly)
• looks very simple
• quoting is confusing (source: http://youtube.com/asdfas is a bug)
• no support for newlines
• needs a custom implementation for each service (upload, processmedia, and scripts are already incompatible...)

json:

• most languages have support built-in, no need to even use a library
• not as human-friendly as the alternatives
• more verbose than the alternatives
• jq is a nice command line tool

yaml:

• weird quoting rules (en is a string, jp, is a string, kr is a string, no is a boolean)

toml:

• seems consistent across languages
• easy enough for humans

I'd like to have one format which all tools support, but temporarily having support for multiple formats in parallel means we can migrate by parts