fix(markdown) Ignore mid_word underscores

Author: danvk

src/languages/markdown.js

@@ -156,8 +156,8 @@ export default function(hljs) {
         end: /\*/
       },
       {
-        begin: /_(?![_\s])/,
-        end: /_/,
+        begin: /(?<![a-zA-Z0-9])_(?![_\s])/,
+        end: /_(?![a-zA-Z0-9])/,
         relevance: 0
       }
     ]

test/markup/markdown/bold_italics.expect.txt

@@ -31,3 +31,6 @@ _ not italic_
 <span class="hljs-quote">&gt; * One (this point is italic)</span>
 <span class="hljs-quote">&gt; * Two</span>
 <span class="hljs-quote">&gt; * Three</span>
+
+No italics mid_word underscores.
+But italics for <span class="hljs-emphasis">_full_word_</span> with underscores.

test/markup/markdown/bold_italics.txt

@@ -31,3 +31,6 @@ _ not italic_
 > * One (this point is italic)
 > * Two
 > * Three
+
+No italics mid_word underscores.
+But italics for _full_word_ with underscores.