add libaribb24 ARIB STD-B24 caption decoder

add libaribb24 ARIB STD-B24 caption decoder

* Outputs ASS lines with basic coloring and font scaling for each
  given region.
* Sets the default style to the resolution of the subtitle plane
  (for example, 960x540 / 36pt font for profile A).
* Has options to:
  * Disable ruby text (which is coded as regions which have
    half-height text in libaribb24).
    Enabled by default as without positioning ruby text only
    confuses as it is usually coded in the beginning of the decoded
    subtitle line.
  * Set the working directory, in which libaribb24 will read
    configuration as well as into which it may save broadcast extra
    symbols as PNG.
    Unset by default.

The unconventional library check can be explained by the library's
current master branch being licensed as LGPLv3, but at the time of
writing the latest official release is still licensed under GPLv3.

Thus, one either has to wait for the following release, or enable
GPLv3.