Docsify must be hosted on a server that supports a default directory
index (i.e. maps `/.../` -> `/.../index.html`).

Some platforms do not support this, however. For example, HTML apps
hosted on the popular game/software platform, Itch.io.

This change supports hosting Docsify off an explicit path file, such as
`/index.html`. It does this by:

 1. Adding handling for paths like `index.html#/blah`, and
 2. Normalising paths with fragments back to markdown paths

For example, `http://example.org/index.html#/blah` would be mapped to
`http://example.org/blah.md`.

This fixes:

docsifyjs#427