themes: new square theme

square is a mix of the existing 'minimal' and 'standard' themes without
any unicode characters that might fail to render in some terminal
emulators.

Signed-off-by: Michael Turquette <mturquette@deferred.io>

themes/square/square.theme.bash

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+prompt() {
+  PS1="${green}\w${reset_color} $(__git_ps1 [${red}%s${reset_color}])$ "
+}
+
+PROMPT_COMMAND=prompt