Begin the process of switching over to using Python-Markdown

This gets us easier extensions from us, and access to useful
extensions that are built in, such as fenced code blocks (that is
github style ``` code blocks).

This is all working as well as the previous code, except for
transclusion, which has yet to be implemented, but is likely to
be implemented in much the same way are the prior lib.

Some important changes:

* plan uris are no longer processed, <http://fdafdsa> is the default
  way, so using that for now. I might fix that if someone like @FND complains
* https://github.com/waylan/Python-Markdown/issues/196
* links get a class of wikilink
* code is broken down into multiple modules to support the extension
  model

A bug I've just realized:

* if wiki_link_base is not set, none of the extension code gets
  added, this duplicates the old behavior but I'm not sure if that
  makes sense. Maybe the name is wrong. Perhaps just 'link_base',
  but then what about backwards compatibility

Todo:

* implement transclusion
* test the newly added (internal) extensions
Get transclusion working.

This is essentially the same code as before, just prepped
a bit differntly:

Transclusion is handled as postprocessor, so our regexp is looking
for a paragraph containing just {{title}} and then doing some
heuristics to determine the correct bag in which to find that tiddler.

As before we render the content using the higher renderer, so we can
mix markdown and other renderable types (such at tiddlywiki) just fine.
pep8 and pyflakes cleanup, plus translcusions fixes

The pyflaking revealed that a tiddler was not including
itself in a sensible fashion when the tiddlyspace context
was available.
Enable simple urlification of https? uris

This borrows heavily from https://github.com/r0wb0t/markdown-urlize
but is considerable less liberal in scope.

Modified tests to make sure that default link style for Markdown
still works.
Annotate an xfail test with info on how to fix

We may not want to fix though, given the content on the
other side of the annotation link.
Sweet, glorious footnotes!

I expect this will get used quite a bit by @FND.

This just tests the functionality provided by the built in
extension, making sure it does in fact work.
fenced code blocks test
add tests for def_list
update docs and readme
Guard against IndexError in transclusion

When using the non-tiddlyspace transclusion regexp, there
will never be a 2nd match.
correctly list dependencies

we don't use utils, but we do use tiddlyweb
Merge pull request #11 from cdent/newlib

Switch to using the other Python Markdown library