Proof of concept: a working prototype with no templates

This commit adds a prototype blog that loads markdown files from the /posts
folder, lists them on the homepage in order of date modified (most recent
first), and parses markdown files into HTML when viewing a blog post.

Needs some serious refactoring to make it tidy, and the addition of mustache
templates to make it look good.
Mpves the `postList` type to a separate file
Adds a `post` struct with a `Slug` method & refactors.

This simplifies code a lot, removing the need for `map[string]interface{}`
and typecasting everywhere. Much cleaner!
Removes the defunct `slug` method from main.go
Adds the correct status codes for not found/internal server error
Adds basic templates for testing. No CSS/classes yet!
Moves Markdown rendering from postList to the Post struct.

Allows markdown to call for content.
Adds mustache rendering for the homepage, post page and error pages.

Need to add styles and the correct theme, now!
Adds initial style to the view blog and homepage
Ensure new posts are checked for on the homepage