Warn if the shell is not supported

If the shell is not recognised as a supported ("bash" and "zsh" for now)
then a warning will be displayed in red.

For example on Mac OS X the shell process is named "-bash" (note the
first character) and is not yet recognised as a valid shell name.

liquidprompt

@@ -884,6 +884,8 @@ prompt_on()
             LP_OLD_PROMPT_COMMAND="$PROMPT_COMMAND"
         elif [[ "$WORKING_SHELL" == "zsh" ]]; then
             LP_OLD_PROMPT_COMMAND="$precmd"
+		else
+			echo "${RED}Shell $WORKING_SHELL not supported $NO_COL"
         fi
     fi
     if [[ "$WORKING_SHELL" == "bash" ]]; then