Modify manifest and association formats for better forward compatibility. (#17)

* Minor language edits.

* Make `app_links` an object instead of an array for better extensibility and foward compatibility.

* Remove suggestion that handshake could also use platform specific assoc. file formats.

* Markdown linting.

* More JSON format changes for extensibility and forwards compat.

* Fixing more lint errors.

* Reformat `app_links` to an array of objects for extensibility and forwards compat.

* Revert `app_links` to `url_handlers` to avoid confusion with `applinks` in the AASA format, until we find a more suitable name.

* Delete `capture_links_exclude_paths` which will not be a part of the sw-launch declarative link capturing proposal.

Author: LuHuangMSFT