Add ITernFileHelper extension point 

This extension point allows adopters to specify multiple tags to be searched in the TernFile when building the javascript text to send to the tern server.

This will be extended in Liferay IDE like this:

```
<extension
          point="tern.eclipse.ide.core.ternFileHelpers">
       <fileHelper
             extraTags="aui:script">
       </fileHelper>
</extension>
```