Add an option to surround the hint character with square brackets
(for example `[a]` rather than `a`). This is the equivalent to the
following `tmux-fingers` configuration option [1] and is useful to make
the hint more visible:
```
set -g @fingers-compact-hints 0
```
[1] - https://github.com/Morantron/tmux-fingers#fingers-compact-hints
Signed-off-by: James O. D. Hunt <jamesodhunt@gmail.com>