Lua scripting / other configuration solution #16

It would be nice to add configuration/scripting to Jet via Lua.

Why Lua instead of a 'static' config format like toml, yaml, json, etc?

• Being able to receive/send jupyter messages via Lua would make it possible to extend the Jet CLI to support kernel-specific features (i.e. comms), or to just execute code in response to other stuff, e.g. the terminal width changing
• Would be fairly straightforward to allow users to implement custom commands, e.g. :restart, %install ..., etc
• More things I haven't thought of yet

Implementation

The fact that we already expose a Lua library complicates things, since mlua doesn't let you expose a library and vender a Lua runtime. So we'd basically need to build in 2 modes:

• If we're building the Lua library, we'd need to piggyback off the host Lua session and reuse that Lua runtime to support Jet's scripting

• If we're building the CLI we'd bundle our own Lua runtime, i.e. Luajit.

Stuff which should be configurable

• Completions window appearance
• Keybindings (e.g. for completions)
• Styling of output from external clients

Stuff which should be scriptable

• Intercept CLI (repl) input/output text. Would have to think about how to handle escapes, though
• Callbacks for jupyter messages (both sent and received)
• Session name. Makes more sense to have this as scriptable than configurable.