Make character_per_seconds correctly deal with tags and introduce text_w...

BTW: is not the commit message too long? Github truncates it.

This ignores tags that srt format allows. It could be done with one regular expression, but I wanted to be explicit in what is ignored (and replace is faster then re, but as it gets called several times, I guess this is overall slower).

SRT is also said to support position coordinates, but there are several ways to write them and I have never actually seen them in the wild, unlike other tags.