Adds a new function in the global namespace to check if the current view
being processed is for an instant article. Handles both the RSS feed and
content which is being processed for the Instant_Articles_Publisher
class. This conditional can be checked in any filters on 'the_content',
'the_title' or elsewhere that need to treat Instant Articles content
separately from web content.

Rather than tracking down every place that might call
Instant_Articles_Post::to_instant_article and wrapping it call to set
the is_instant_article conditional, it's much simpler to just hook those
settings on the existing actions `instant_articles_before_transform_post`
and `instant_articles_after_transform_post`.

Rename `is_instant_article` to `is_transforming_instant_article`, to
better indicate that it will return true when a tranformation is being
processed.

Since these hooks can be modified in plugins, it's safer to set the
status of is_transforming_instant_article() directly in the
`Instant_Articles_Post::to_instant_article` method.