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.

Fixed spellings : 
1. shebang
2. identification
Added 'the' before 'said'
Fixed vertical line column size to 79


Fixed failing tests : Removed trailing spaces

* Python syntax upgraded using `pyupgrade --py3-plus`
* Travis no longer uses `sudo`. See https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

See #760 for Python Version Support Timeline and related dicussion.

Pygments added the `wrapcode` option in version 2.4. Users need to have 2.4+ installed to see the change. However, as earlier versions accepted arbitrary keywords passed to the HTMLFormatter, no error will be raised if the user has an older version of Pygments installed.

Fixes #725

In the past, most of the config was defined using globals. Today all of 
the config is held on the class instance. Therefore, the `md_globals` 
parameter is no longer necessary.

Previously, instances of the Markdown class were represented as any one 
of 'md', 'md_instance', or 'markdown'. This inconsistency made it 
difficult when developing extensions, or just maintaining the existing 
code. Now, all instances are consistently represented as 'md'.

The old attributes on class instances still exist, but raise a 
DeprecationWarning when accessed. Also on classes where the instance was 
optional, the attribute always exists now and is simply None if no 
instance was provided (previously the attribute wouldn't exist).

All processors and patterns now get "registered" to a Registry.
Each item is given a name (string) and a priority. The name is for
later reference and the priority can be either an integer or float
and is used to sort. Priority is sorted from highest to lowest. A 
Registry instance is a list-like iterable with the items auto-sorted 
by priority. If two items have the same priority, then they are 
listed in the order there were "registered". Registering a new 
item with the same name as an already registered item replaces
the old item with the new item (however, the new item is sorted by
its newly assigned priority). To remove an item, "deregister" it by 
name or index.

A backwards compatible shim is included so that existing simple
extensions should continue to work. DeprecationWarnings will 
be raised for any code which calls the old API.

Fixes #418.