Support tri-state "format_url" instead of boolean "use_directory_urls".

#### url_format

This setting controls the format of the URLs (and filenames for a static "build").  The options for `url_format` are `directory` (default), `index` or `file`.  This parameter replaces the old `use_directory_urls` flag.

The behavior for `url_format` = `directory` is the default and matches the old `use_directory_urls` = `True`.  This generates clean URLs, although it has the problem that a static build does not generate links that work when browsing off a file.

Also, this format breaks relative links to images and other media files which are stored within the `.md` document tree.  E.g. if `system.md` contains an image `<img src="a.jpg">` where `a.jpg` is in the same directory as `system.md`, then this image will not display with `url_format` = `directory` either in static `build` or dynamic `serve` modes using `directory` format URLs.  Instead inside the `system.md` you must now refer to the image as `<img src="../a.jpg">`, due to the trailing `/` which is added to the navigation link.  If you wish to preserve relative image links after processing, you may want to consider `url_format` = `file`.

Here is the behavior for `url_format` = `directory`

Source file  | Generated HTML       | Navigation Links
------------ | -------------------- | ------------------------
index.md     | index.html           | /
api-guide.md | api-guide/index.html | /api-guide/
about.md     | about/index.html     | /about/
dir/file.md  | dir/file/index.html  | /dir/file/

The behavior for `url_format` = `index` matches what the old `use_directory_urls` = `False` claimed to do. This generates a static build which does work when browsing from a file system.  However it still breaks relative links to images and other media files which are stored within the `.md` document tree.

Source file  | Generated HTML       | Navigation Links
------------ | -------------------- | ------------------------
index.md     | index.html           | /index.html
api-guide.md | api-guide/index.html | /api-guide/index.html
about.md     | about/index.html     | /about/index.html
dir/file.md  | dir/file/index.html  | /dir/file/index.html

The behavior for `url_format` = `file` simply translates the `.md` suffix into `.html`. This generates a static build which does work when browsing from a file system, and preserves relative links to images and other media files which are stored within the `.md` document tree.

Source file  | Generated HTML       | Navigation Links
------------ | -------------------- | ------------------------
index.md     | index.html           | /index.html
api-guide.md | api-guide.html       | /api-guide.html
about.md     | about.html           | /about.html
dir/file.md  | dir/file.html        | /dir/file.html

**default**: `directory`