Includes new configuration option `git-repository-edit-baseurl` for
supporting non-GitHub repository layouts.

…dependent of git_repository_url.

The `IndexPreprocessor` rewrites the path for files
named `README.md` to be `index.md`. This breaks the edit link
in some circumstances.

To address this issues, the `Chapter` struct has now a new attribute
called `source_path`. This is initialized with the same value as
`path`, but is never ever changed.

Finally, the edit link is built by using the `source_path` rather
than the `path`.

Add a working example of the edit links feature to the examples

Add "edit" links to the pages of mdbook own book

Change the name of the git-repository-edit-url-template to be more
generic: `edit-url-template`

Signed-off-by: Flavio Castelli <fcastelli@suse.com>