The logic for the current regex for strong/em and em/strong was sound,
but the way it was implemented caused some unintended side effects.
Whether it is a quirk with regex in general or just with Python’s re
engine, I am not sure.  Put basically `(\*|_){3}`  causes issues with
nested bold/italic. So, allowing the group to be defined, and then
using the group number to specify the remaining sequential chars is a
better way that works more reliably `(\*|_)\2{2}.  Test from issue #365
was also added to check for this case in the future.

This issue was discovered when dealing with nested inlines.  In
treeprocessors.py it was incorrectly handling tails.  In short, tail
elements were being inserted earlier than they were supposed to be.

In order to fix this, the insertion index should be incremented by 1 so
that when the tails are inserted into the parent, they will be just
after the child they came from.

Also added a test in nested-patterns to catch this issue.