lib: Add _has_colors function and use it in log output

lib/appearance.bash

@@ -1,5 +1,15 @@
 #!/usr/bin/env bash
 
+function _has_colors()
+{
+    # Check that stdout is a terminal
+    test -t 1 || return 1
+
+    ncolors=$(tput colors)
+    test -n "$ncolors" && test "$ncolors" -ge 8 || return 1
+    return 0
+}
+
 # colored ls
 export LSCOLORS='Gxfxcxdxdxegedabagacad'