Add a way to provide a composite context with <% include %>

[tifosiblack]

Hello Vadim,

I propose this pull request, in order to be able to pass a 'this' extended context through an include.

This way I'm able to include reusable component like that and still be able to use global headers, like translation, defined into 'this' :

<% include 'runtime/components/media', @, { media: @exercise.media } %>
<% include 'runtime/components/media', @, { media: @other.media } %>

Context is merge into a new object in this case. The behaviour is unchanged in other cases.