configurable graph styles

[ttytm]

I'm clearly missing some characters in FantasqueSansMono Nerd Font or, on alacritty

Originally posted by @0323pin in #26

After some testing, we concluded that the glyphs used for the graph simply did not work in alacritty under BSD.
Rather than spend more time debugging, I tested other Unicode symbols for the graph to work around this problem. They don't have as many levels, but work with alacritty.

Some simple confugrable variants for the graph will be added in the next minor release.