implements custom templating system

this adds the foundation for a custom templating system. there are a couple of assumptions this system is based on…

1) that, while users may want to customize the templates without hacking the core template files, they will probably never want (or need) to have *multiple* custom templates. This is different from the WordPress themeing system, where you can have multiple themes installed but only one active theme -- with this system, you can only have one custom theme.
2) the custom template requires at least a style.css and that that custom file is located in the /template/custom directory. Any of the other core template files can be overridden. New templates could (potentially) be added by including those files in the template files themselves (e.g. the comments template in https://gist.github.com/jazzsequence/5144026). Any template files not included in the /custom directory will default to the core template files.

In theory, this allows the flexibility of creating or customizing the templates without having to hack the core files, and only customizing/editing those templates that *need* to be changed (e.g. you can leave the ones you aren't going to touch).
adds strikethrough support

this is a quick hack to support `~~` for strikethroughs, e.g. ~~strike~~. The hack part comes into play if your ~~ is at the end of a line, like: ~~this is a strikethrough.~~

In that case, you need to add an extra space at the end of the line so the `str_replace` function sees `~~ ` and replaces it with a `</del>`. If I get better at crazy preg_match or preg_split or other regular expression functions like that, this should be updated to support it more natively. but for now, this is probably more of an edge case just for me, , this should be update Mou supports ~~ as Strikethroughs.
Gives the stylesheet it's own variable

This way the stylesheet path can be called independently and, therefore, be different based on whether a custom template is being used. This eliminates the need for a custom `index.php` file if you're using a custom template.
adds the new stylesheet_dir variable to the index template

if a custom template is defined, this will point to the custom template's style.css, otherwise it will point to the default style.css
Merge pull request #17 from jazzsequence/master

Implement a custom template system