subparse: Support for parsing SubRip font tags

The following markup: <font color="#notation or color name here">...</font> is converted to a Pango span tag (<span color="...">...</span>).

A dedicated attribute-matching regex was added to the parser, in order to detect quoted attributes. It wasn't needed before most likely because the initial WebVTT format supported by this parser didn't define quoted attributes in its grammar. The WebVTT parsing clearly needs a refresh though...