Add -p/--pretty option to swaymsg

This new option forces pretty (non-raw/non-JSON) output. By default, when
not using a tty, swaymsg outputs using the "raw" format. This makes it
impossible to, for example, pipe the pretty output to a pager such as
`less` since piping does not use a tty.

The new -p/--pretty option gives the user explicit control over the output
format while retaining the default tty-dependent behavior.

Signed-off-by: Peter Grayson <pete@jpgrayson.net>

completions/bash/swaymsg

@@ -21,6 +21,7 @@ _swaymsg()
 
   short=(
     -h
+    -p
     -q
     -r
     -s
@@ -30,6 +31,7 @@ _swaymsg()
 
   long=(
     --help
+    --pretty
     --quiet
     --raw
     --socket