Correct the -m flag in the usage text

The usage text incorrectly says that the -m flag is -t. Make it correct
and point out that the flag's argument specifies the displayed message.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Use no title by default

The dialog may have a default title, which doesn't necessarily make much
sense. Clear it and set it to the empty string.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Make it possible to set title

Add an option that allows the user to specify the title. Fall back to
the empty string if no title is specified.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>