Stop less paging from clearing the screen

The `X` option's disabling of termcap init and deinit strings allows us to
retain the contents of less on screen while going back to a prompt. Good when
looking at man pages and git history.

zsh/custom/less.zsh

@@ -4,4 +4,5 @@
 # i = searches ignore case, unless there are upper case characters
 # r = display raw control characters (Ctrl-A is ^A)
 # M = verbose `less` prompt
-export LESS='-iRM'
+# X = disable termcap init/deinit strings - sometimes these clear the screen.
+export LESS='-iRMX'