Add <content></content> to allow content injection in paper-input

[mobula]

A small change that allows injecting custom content.
See for instance paper-tags-typeahead prototype created by combining paper-typeahead with PolymerEl/paper-tags.

[cheonhyangzhang]

@mobula
Can you explain what's the use case for this?

[mobula]

My use case was to combine typeahead with a tags component such as paper-tags. you type and get suggestions as if you were using typeahead, but if you select the component, it becomes a tag. and so on.

<paper-typeahead-tags>
   <paper-typeahead-input>
      <paper-tags>...</paper-tags>
   </paper-typeahead-input>
</paper-typeahead-tags>