Adding support for libass kerning extension

[Witiko]

The libass library supports the Kerning parameter in the [Script Info] section of SubStation Alpha subtitles, which enables the kerning of fonts. Kerning greatly improves the letterspacing in subtitles:

Although the extension is non-standard, it is helps to produce more readable subtitles. Is there either any intention of adding this extension to xy-VSFilter or a reason to reject a pull request that would implement this functionality?

This issue is also discussed at Aegisub/Aegisub#164.