New naming convention for entries so generated files are structured within folders

Entries should follow the naming convention `YYYY-MM-DD-post-title.md`. Generated files are then structured within the following folder structure `YYYY/MM/DD/post-title.html`.

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Having this naming convention will give the ability to define a permalink structure within the `generate.php` file, for example `define('MB_PERMALINK','/:year/:title');`

I'll send another pull request when I've got this working.