Issue #365 Bold/Italic nesting fix

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.

markdown/inlinepatterns.py

@@ -101,8 +101,8 @@ def build_inlinepatterns(md_instance, **kwargs):
 ESCAPE_RE = r'\\(.)'                             # \<
 EMPHASIS_RE = r'(\*)([^\*]+)\2'                    # *emphasis*
 STRONG_RE = r'(\*{2}|_{2})(.+?)\2'                      # **strong**
-EM_STRONG_RE = r'(\*|_){3}(.+?)\2(.*?)\2{2}'            # ***strongem*** or ***em*strong**
-STRONG_EM_RE = r'(\*|_){3}(.+?)\2{2}(.*?)\2'            #  ***strong**em*
+EM_STRONG_RE = r'(\*|_)\2{2}(.+?)\2(.*?)\2{2}'            # ***strongem*** or ***em*strong**
+STRONG_EM_RE = r'(\*|_)\2{2}(.+?)\2{2}(.*?)\2'            #  ***strong**em*
 SMART_EMPHASIS_RE = r'(?<!\w)(_)(?!_)(.+?)(?<!_)\2(?!\w)'  # _smart_emphasis_
 EMPHASIS_2_RE = r'(_)(.+?)\2'                 # _emphasis_
 LINK_RE = NOIMG + BRK + \

tests/misc/nested-patterns.html

@@ -5,4 +5,6 @@
 <strong><a href="http://example.com"><em>link</em></a></strong>
 <strong><a href="http://example.com"><em>link</em></a></strong>
 <a href="http://example.com"><strong><em>link</em></strong></a></p>
-<p><strong><em>I am <strong><em>italic</em> and</strong> bold</em> I am <code>just</code> bold</strong></p>
+<p><strong><em>I am <strong><em>italic</em> and</strong> bold</em> I am <code>just</code> bold</strong></p>
+<p>Example <strong><em>bold italic</em></strong> on the same line <strong><em>bold italic</em></strong>.</p>
+<p>Example <strong><em>bold italic</em></strong> on the same line <strong><em>bold italic</em></strong>.</p>

tests/misc/nested-patterns.txt

@@ -7,3 +7,7 @@ __[*link*](http://example.com)__
 [***link***](http://example.com)
 
 ***I am ___italic_ and__ bold* I am `just` bold**
+
+Example __*bold italic*__ on the same line __*bold italic*__.
+
+Example **_bold italic_** on the same line **_bold italic_**.