Allow specifying the full module name for extensions.
Allow specifying the full module name for extensions.

Before this patch, it was possible to pass a list of extensions and
parameters to use when instantiating a new Markdown instance, but it was
not possible to give a full module name; the extension had to be a
submodule in markdown.extensions.

Now we allow giving a full module path, to make it easy to use custom
extensions which are not bundled within the markdown package. The
previous behaviour has been retained when there is no '.' in the
extension name.

waylan/Python-Markdown,waylan/Python-Markdown,me-and/Python-Markdown,evertqin/Python-Markdown,gogobook/Python-Markdown,fernandezcuesta/Python-Markdown,joachimneu/Python-Markdown,fernandezcuesta/Python-Markdown,zestedesavoir/Python-ZMarkdown,me-and/Python-Markdown,Situphen/Python-ZMarkdown,dataquestio/Python-Markdown,gogobook/Python-Markdown,zestedesavoir/Python-ZMarkdown,zestedesavoir/Python-ZMarkdown,joachimneu/Python-Markdown,cyisfor/Python-Markdown,evertqin/Python-Markdown,evertqin/Python-Markdown,Situphen/Python-ZMarkdown,dataquestio/Python-Markdown,Situphen/Python-ZMarkdown,cyisfor/Python-Markdown,joachimneu/Python-Markdown,dataquestio/Python-Markdown,gogobook/Python-Markdown,cyisfor/Python-Markdown,me-and/Python-Markdown,fernandezcuesta/Python-Markdown