Extend plugin info to allow adding header tags

This allows a plugin to add javascript or css tags in the head of the main applications layout. 
Due to the use of turbolinks it is not always possible to add javascript or css inside a plugins partial / hook because there can be some issues with javascript loading order. 

The implementation is heavily inspired by ```add_view_hook```, which unfortunately could not be used because it requires a controller name

You can add javascript tags like this (it's also possible to pass in a partial or proc)
```
add_header_tags do
    javascript_include_tag "myjavacriptlib"
end
```