If pygments is installed and the version doesn't match the expected version.
then any relevant tests will fail. To avoid failing tests due to different
output by pygments, those tests will be skipped. The pygments tox env
sets the `PYGMENTS_VERSION environment variable, so that env will always
run those tests against the expected version.

Closes #1030

Previously, we'd unescape both `>` and `>` to the same
string because we were running the & => & replacement first.
By changing the order of this replacement, we now convert:

`> >` => `> >`

as expected.

Fixes #988.

* Add `language-` prefix to output when syntax highlighting is
  disabled for both codehilite and fenced_code extensions.
* Add `lang_prefix` config option to customize the prefix.
* Add a 'pygments' env to tox which runs the tests with Pygments
  installed. Pygments is locked to a specific version in the env.
* Updated codehilite to accept any Pygments options.
* Refactor fenced code attributes.
   - ID attr is defined on `pre` tag.
    - Add support for attr_list extension, which allows setting arbitrary 
      attributes. 
    - When syntax highlighting is enabled, any pygments options can
       be defined per block in the attr list.
    - For backward compatibility, continue to support `hi_lines` outside
      of an attr_list. That is the only attr other than lang which is allowed
      without the brackets (`{}`) of an attr list. Note that if the brackets
      exist, then everything, including lang and hl_lines, must be within
      them.
* Resolves #775. Resolves #334. Addresses #652.