Allow users to change the character used for the progress graph

[ifd3f]

Is your feature request related to a problem? Please describe.

Some fonts render braile characters differently, so the graph is ugly.

Describe the solution you'd like

A user preferences file probably located at ~/.config/caligula.toml and a way to change the Marker of the progress graph. If we're feeling fancy, we could even add a caligula config subcommand to update it, like git has?

Describe alternatives you've considered

N/A

Additional context

N/A