Create a supported mechanism for users to bring in their own settings

[pyllyukko]

• It needs to be something clean and classy, and not just cat 1.js 2.js >3.js
• Maybe some git branch trickery?
• On the IRC, m4 was mentioned 🙂
  • There was also discussion about "tagging" each setting, so the user can choose which categories/groups of settings get pulled into the final user.js
• Considering, that we already have kind of "format" in the .js (sections, PREF, NOTICE & links)... we could use some standard format e.g. YAML or json to describe the settings and then have a preprocessor to produce user.js & README (instead of completely custom parser)
  • We should also have attribute that describes which FF versions are affected to tackle Drop support for older Firefox versions #251
• Remember .gitignore
• Some "standard" user.js should still be readily available in the repo for those who don't wish to mess around with make and other CLI stuff

Additional notes:

• Would help to produce files like this
• Relates to Modular user.js instead of a monolith #394