[w3c/manifest] Parsing image purpose. (#516)

This is on top of #515.

@marcoscaceres PTAL :smile: 
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/516

-- Commit Summary --

  * When purpose is invalid, return set with "any" instead of undefined (fixes #513).
  * Parse "purpose" when parsing an image (fixes #511).

-- File Changes --

    M index.html (9)

-- Patch Links --

https://github.com/w3c/manifest/pull/516.patch
https://github.com/w3c/manifest/pull/516.diff

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