Modify HTTPServer not to get template for constructor.

Instead of getting the template, just initialise one in the constructor.
Implement static file serving.

Now `orange-cat` can serve static files. Basically, it will find the
file from the directory where the Markdown file exists, and start to
traverse parents until the static file's found or it gets to the root
directory.
Use a custom CSS if it exists.

The CSS is loaded from `$HOME/.orange-cat.css`.
Update README.md

Add the `Custom CSS` section.
Release v0.2.0

- Static file serving
- Custom CSS