Refactor Extension loading (#627)

Deprecated naming support is removed:

* Removed special treatment for modules in `markdown.extensions`
* Removed support for `mdx_` prefixes.

Support for Entry Point names added:

Support for "short names" are now implemented with entry points. 
Therefore all the users who call extension names as `toc` will not 
get errors as the builtin extensions all have entry points defined 
which match the old "short names" for modules in 
`markdown.extensions`. The benefit is that any extension can offer
the same support without requiring the user to manually copy a file 
to that location on the file system (way to many extension authors 
have included such instructions in their installation documentation).

The one odd thing about this is that we have been issuing a 
DeprecationWarning for short names and now they are fully supported 
again. But I think it's the right thing to do.

Support for using dot notation is not removed. After all, it was never 
deprecated. And we shouldn't "force" entry points. There are plenty of 
reasons why users may not want that and not all of them can be 
resolved by using class instances instead.

All of the following ways to load an extension are valid:

    # Class instance
    from markdown.extensions.toc import TocExtension
    markdown.markdown(src, extensions=[TocExtension()]

    # Entry point name
    markdown.markdown(src, extensions=['toc'])

    # Dot notation with class
    markdown.markdown(src, extensions=['markdown.extensions.toc:TocExtension'])

    # Dot notation without class
    markdown.markdown(src, extensions=['markdown.extensions.toc'])

1 files changed, 3 insertions, 3 deletions

diff --git a/docs/extensions/meta_data.md b/docs/extensions/meta_data.md
index 36d5e7a..29f3e0b 100644
--- a/docs/extensions/meta_data.md
+++ b/docs/extensions/meta_data.md
@@ -57,8 +57,8 @@ by Markdown.
 Usage
 -----
 
-See [Extensions](index.md) for general extension usage, specify
-`markdown.extensions.meta` as the name of the extension.
+See [Extensions](index.md) for general extension usage. Use `meta` as the name
+of the extension.
 
 Accessing the Meta-Data
 -----------------------
@@ -67,7 +67,7 @@ The meta-data is made available as a python Dict in the `Meta` attribute of an
 instance of the Markdown class. For example, using the above document:
 
 ```pycon
->>> md = markdown.Markdown(extensions = ['markdown.extensions.meta'])
+>>> md = markdown.Markdown(extensions = ['meta'])
 >>> html = md.convert(text)
 >>> # Meta-data has been stripped from output
 >>> print html