Account for HTML tags order and overlapping

[retifrav]

Current implementation of subtitles validation (#10) doesn't account for order and overlapping of HTML tags, so for example this title won't fail validation:

2
00:00:34,840 --> 00:00:36,840
That is clearly </u>wrong<u>,
even though open/close tags count is fine.

neither will this one:

3
00:00:40,760 --> 00:00:43,760
Tags <font color"green">are <b>incorrectly</font> overlapping</b>,
but still the count of open/close is fine, so no problem is raised.