Fix the lost tail issue in inlineprocessors.

See #253. Prior to this patch, if any inline processors returned an element
with a tail, the tail would end up empty. This resolves that issue and will
allow for #253 to be fixed. Thanks to @facelessuser for the work on this.
Better nested STRONG EM support.

Fixes #253. Thanks to @facelessuser for the tests. Although I removed
a bunch of weird ones (even some that passed) from his PR (#342). For
the most part, there is no definitive way for those to be parsed. So
there is no point of testing for them. In most of those situations,
authors should be mixing underscores and astericks so it is clear
what is intended.