Ignore underscores in the middle of words.

test/original/emphasis_and_strong.unit

Index: test/original/emphasis_and_strong.unit
diff --git a/test/original/emphasis_and_strong.unit b/test/original/emphasis_and_strong.unit
index 5e67fc043b130160ff4744451d8b8edf3cd7eafc..90aea56ab58cd908b4acb62d29342fd4d2429b98 100644
--- a/test/original/emphasis_and_strong.unit
+++ b/test/original/emphasis_and_strong.unit
@@ -70,3 +70,19 @@ a * b * c _ d _ e
 
 <<<
 <p><em>a _b </em>c<em> d_ e</em></p>
+>>> in the middle of a word
+a_b_c a__b__c a*b*c a**b**c
+<<<
+<p>a_b_c a__b__c a<em>b</em>c a<strong>b</strong>c</p>
+>>> prefixing a word
+_a_b __a__b *a*b **a**b
+<<<
+<p>_a_b __a__b <em>a</em>b <strong>a</strong>b</p>
+>>> suffixing a word
+a_b_ a__b__ a*b* a**b**
+<<<
+<p>a_b_ a__b__ a<em>b</em> a<strong>b</strong></p>
+>>> spanning words
+_a_b c_d_ __a__b c__d__ *a*b c*d* **a**b c**d**
+<<<
+<p><em>a_b c_d</em> <strong>a__b c__d</strong> <em>a</em>b c<em>d</em> <strong>a</strong>b c<strong>d</strong></p>