title: Pygments error: no lexer for alias - Underscores in code block
body: <p>I'm working on a github-pages project with all the usual suspects.  I'm trying to switch my markdown from kramdown to redcarpet to better support GFM for local development.  I have an existing project that has just one troublesome section of markdown.  It's a single-line code block with a variable name containing underscores.</p>
  
  <pre><code>```function_field_name```
  </code></pre>
  
  <p>_config.yml</p>
  
  <pre><code>markdown: redcarpet
  
  redcarpet:
    extensions: ["tables", "autolink", "strikethrough", "space_after_headers", "with_toc_data",  "no_intra_emphasis", "fenced_code_blocks"]
  
  highlighter: pygments
  safe: true
  </code></pre>
  
  <p>I get the following error:</p>
  
  <blockquote>
    <p>Conversion error: There was an error converting 'queries.md'. jekyll
    2.2.0 | Error:  Traceback (most recent call last):   File "/Users/shawnjohnson/.rvm/gems/ruby-2.1.1/gems/pygments.rb-0.6.0/lib/pygments/mentos.py",
    line 303, in start
        res = self.get_data(method, lexer, args, kwargs, text)   File "/Users/shawnjohnson/.rvm/gems/ruby-2.1.1/gems/pygments.rb-0.6.0/lib/pygments/mentos.py",
    line 171, in get_data
        res = self.highlight_text(text, lexer, formatter_name, args, _convert_keys(opts))   File "/Users/shawnjohnson/.rvm/gems/ruby-2.1.1/gems/pygments.rb-0.6.0/lib/pygments/mentos.py",
    line 122, in highlight_text
        lexer = self.return_lexer(lexer, args, kwargs, code)   File "/Users/shawnjohnson/.rvm/gems/ruby-2.1.1/gems/pygments.rb-0.6.0/lib/pygments/mentos.py",
    line 79, in return_lexer
        return lexers.get_lexer_by_name(lexer, **inputs)   File "/Users/shawnjohnson/.rvm/gems/ruby-2.1.1/gems/pygments.rb-0.6.0/vendor/pygments-main/pygments/lexers/<strong>init</strong>.py",
    line 98, in get_lexer_by_name
        raise ClassNotFound('no lexer for alias %r found' % _alias) ClassNotFound: no lexer for alias 'function_field_name```' found</p>
  </blockquote>