srt export: add per line <italic/bold/etc> marking


Currently srt italic subtitles are saved in this format:

00:00:34,551 --> 00:00:44,550
<i>line one text
line two text</i>

Although this is valid and properly recognized by media players, it confuses aegisub application. When aegisub is used to create ass format subtitles from an srt file using the previous formating, it results in upper line only italic typing and bottom line regular typing.

This behavior is bypassed by using this format:

00:00:34,551 --> 00:00:44,550
<i>line one text</i>
<i>line two text</i>

If I remember correctly the latter format was used in the passed, but I am not sure. I don't know if there was a reason for changing it in the first place, so if it was, an option to choose the proper format would be great.

Be careful not to mess subtitles with only one italic line of the two.