Add fish_indent --check

Similarly to many other formatters, fish_indent should have a -c/--check option that will tell the user if the fish file is formatted as fish_indent would like.

This is currently achievable with fish_indent $file | diff - $file, but I think it should be added as a standard option.

_{fish, version 3.1.2
Linux archlabs 5.7.12-arch1-1 #1 SMP PREEMPT Fri, 31 Jul 2020 17:38:22 +0000 x86_64 GNU/Linux
xterm-256color
Clean fish}