Multimap remastered

[pgte]

@hij1nx and @3rd-Eden, I need your input on this:

There is only one issue with the merge, and it's related with my fix.

Test 14 and 15 also test for nesting, but when the inner iteration starts the nesting is not done using the "mapping" mode.

Yours: https://github.com/flatiron/plates/blob/master/test/api-test.js#L149 Mine: https://github.com/pgte/plates/blob/multimap-remastered/test/api-test.js#L150

The fix I did reuses the mapping inside each iteration, which I think is correct, but for these cases activate the mapping mode, and the user has to input the missing mappings.

[travisbot]

This pull request passes (merged b0515b8b into 5450c28f).

[pgte]

The fixes on PR 81 are still valid:

---

Several fixes related to mapping the same attribute name and value, using collections and nesting.

• Fixed the case where to mappings for the same attribute and value existed where one changed the tag content and the other updated the attribute. In this case, only when the attribute one was declared last it worked. Changing the order of the declarations made it work. The fix orders the mappings so that the order of the declaration doesn't matter.
• Testing if there are conflicting mappings (updating the body for the same attribute and same value).
• Nested objects work with map directives.
• Arrays now work with map directives.
• Added test for two attributes update on the same tag.
• Added test for two attributes update plus body update on the same tag.
• Added test cases for all these fixes.