[WIP] fix(main): don't require file argument

[Cogitri]

It's entirely reasonable to want to open an empty view, e.g. to create a new document

I guess this also needs a way to specify the filename during save though.

[Cogitri]

Hm, I guess we'd also need to fix handling input from stdin for this? If I run cat README.md | xi-term with this I just get the following:

error: Failed to initialize the TUI
caused by: Failed to initialize the TUI
backtrace: 

But if I just invoke xi-term it works.