config: add `<core><promptCommand>`

config: add `<core><promptCommand>`

...to enable configuration of the action prompt command.

Also set some better defaults for labnag.

The new default command is:

    labnag \
        --message '%m' \
        --button-dismiss '%n' \
        --button-dismiss '%y' \
        --background '%b' \
        --text '%t' \
        --border '%t' \
        --border-bottom '%t' \
        --button-background '%b' \
        --button-text '%t' \
        --border-bottom-size 1 \
        --button-border-size 3 \
        --timeout 0

...where the conversion specifiers are defined as follows:

    %m: the `<prompt>` message option
    %n: _("No")
    %y: _("Yes")
    %b: osd.bg.color
    %t: osd.label.text.color

This config options also enables the use of a different dialog client, for
example like this:

    <core>
      <promptCommand>zenity --question --text="%m"</promptCommand>
    </core>