Feature request: Support for custom plugins

[tadejg]

This may be similar to #90, but a bit more generalized.

I want to write a more complete integration for Trello which will support two way sync. However, I'm not familiar enough with Haskell to contribute to this project myself and most users probably wouldn't need the extra feature, so something like a plugin system could be very beneficial.

I'm aware development is currently paused and this is probably a relatively large feature, but I thought I'd open the issue on the off chance someone is interested in working on this or development resumed at some point.

Some spitball ideas:

• I believe Haskell should support loading DLLs/SOs, so maybe this could be a reasonable way to go about loading plugins. It would also lift the restriction of using Haskell. Bindings for scripting languages could also be written (e.g. JS, Python, Bash, ...)
• Plugins could modify the UI, keybinds, and/or parser/data source
• Alternatively, plugins could be written in a language like Lua (sort of like Neovim)