Add underline 'u' support in extras

lib/markdown2.py

@@ -1153,6 +1153,9 @@ def _run_span_gamut(self, text):
         if "strike" in self.extras:
             text = self._do_strike(text)
 
+        if "underline" in self.extras:
+            text = self._do_underline(text)
+
         text = self._do_italics_and_bold(text)
 
         if "smarty-pants" in self.extras:
@@ -1954,6 +1957,11 @@ def _do_strike(self, text):
         text = self._strike_re.sub(r"<strike>\1</strike>", text)
         return text
 
+    _ins_re = re.compile(r"--(?=\S)(.+?)(?<=\S)--", re.S)
+    def _do_underline(self, text):
+        text = self._ins_re.sub(r"<u>\1</u>", text)
+        return text
+
     _strong_re = re.compile(r"(\*\*|__)(?=\S)(.+?[*_]*)(?<=\S)\1", re.S)
     _em_re = re.compile(r"(\*|_)(?=\S)(.+?)(?<=\S)\1", re.S)
     _code_friendly_strong_re = re.compile(r"\*\*(?=\S)(.+?[*_]*)(?<=\S)\*\*", re.S)

test/tm-cases/ins.html

@@ -0,0 +1 @@
+<p>This is some <u>underline</u> text.</p>

test/tm-cases/ins.opts

@@ -0,0 +1 @@
+{"extras": ["underline"]}

test/tm-cases/ins.tags

@@ -0,0 +1 @@
+extras underline

test/tm-cases/ins.text

@@ -0,0 +1 @@
+This is some --underline-- text.