first pass at adding in some styling for code and pre blocks

source sans pro is already being called from google webfonts, so applying that to `pre` and `code` blocks. Also adding some letter spacing and margins to make it a bit more readable, particularly in `<code>` blocks that are inline with other content.
transforms -- into &mdash;

emdashes are prettier. This adds a new function to str_replace `--` with `&mdash;` and sets it with a priority of 55, so it gets fired right after the bolds and italics.
Merge pull request #15 from jazzsequence/master

First pass at adding some css to handle pre and code tags.