Expose default_features as class-level attributes

This makes it possible to add feature types to subclasses of BlockLexer and InlineLexer, without having to copy-paste the methods where their default feature orders are defined.

Background: We're investigating using mistune for rendering markdown in IPython notebooks (ipython/ipython#6028). One of our requirements is to pass through code in $-delimited mathematical expressions (`$inline$` and `$$block$$`), without trying to render it as markdown (they are rendered by [mathjax](http://www.mathjax.org/) in the browser). At present, we do this by replacing all such expressions with numbered tokens like `@@1@@`, sending it to a renderer, and then filling them back in in the HTML. I hope that by using mistune, we can have a more elegant solution where the parser itself knows about these expressions. I think I can see how to do this, but I need a way to add a feature to the lexers.