Internal relative links not work

[theowenyoung]

Hi, Thanks for the great tool!

It seems internal link not works, for example:

[A](./a.md)

and I found @dieterplex forked branch https://github.com/dieterplex/mdbook-epub/tree/fix-local-md-link can work fine.

@dieterplex would you like merge it to upstream?

[dieterplex]

Context: rust-lang/mdBook#431
So I left #53 closed sometimes ago. 😃

[theowenyoung]

Thanks I see.