fix child exit status

The status returned by pclose() is a combination of the child pid and
its exit status. The actual exit code should be checked with
WEXITSTATUS().
add support for short_text and color

Now a block command may return 3 lines. If so, they will be:

  1. the full_text
  2. the short_text
  3. the color

For example, a script setting a full_text in blue but no short_text
would look like:

    echo "Here's my label"
    echo
    echo \#0000FF