Brush mode

[newsch]

Brush Mode: paint with a character.

┌─────────────────────────────────────────────────┐
│BBB  RRR  U  U SSSS H  H                         │
│B  B R  R U  U S    H  H                         │
│BBB  RRR  U  U SSSS HHHH                         │
│B  B R R  U  U    S H  H                         │
│BBB  R  R UUUU SSSS H  H                         │
│                                                 │
│M  M OOOO DDD  EEEE                              │
│MMMM O  O D  D E                                 │
│M  M O  O D  D EEE                               │
│M  M O  O D  D E                                 │
│M  M OOOO DDD  EEEE                              │
│                                                 │
│                                                 │
│                                                 │
├─────────────────────────────────────────────────┤
│state: OFF     brush: 'E' (Press ENTER to toggle)│
│                                                 │
└─────────────────────────────────────────────────⏎ 

[newsch]

One twist I found: enabling mouse support hijacks the normal method of highlighting for copy-pasting. On my machine with gnome-terminal I need to SHIFT-click to highlight lines and SHIFT-CTRL-click to block-select.

[newsch]

@MatthewBeaudouinLafon @labseven you should try out mouse support on your machines and see if it works there. I'm not sure how flexible this solution is.

[newsch]

Rebased the mode picker stuff into #45 to separate things out.

[newsch]

In a fun twist of fate, util.h now throws unused warnings about _log_unused when DEBUG IS defined.

[newsch]

Fixed by declaring the function as inline.

[newsch]

Whoops, I forgot to rebase this first.

[newsch]

AND I didn't push the update for MacOS. Oof.