Added: support for new indexer features

Please see the whole explanation in https://github.com/cozy/cozy-data-indexer/pull/12

Please note it's not built, so tell me if the review is okay so I can build to let Travis pass the tests.
mapped values are what I explained during the meeting: some fields cannot be indexed as-is because they have an advanced type, say an array of strings. The developer can then define a map function to transform the array of string into something more basic (like a string with all items inside)