Don't require closing tag for tags that don't need it.
Don't truncate inside an HTML comment!
Do truncation by actual character count.

Using substr can cut a multi-byte character in half.

Also, if a long post with many multi-byte characters was reported, then
the mod interface would temporarily extend the body_truncate_char
setting to be sure to cover all of the *characters* in the report, but
this function would interpret body_truncate_char as a number of *bytes*,
so sometimes the end of the report's appended html would be cut off.
Merge pull request #91 from Macil/truncfixes

Post Truncation fixes