Add new filter so that other plugins can override widget context logic checks

Use case:

A user might like the 'single post' checkbox to only apply to posts (and not other custom post types).

Currently it would apply to any custom post type.

With this new filter, the default behaviour doesn't change.

However following code could be used in another plugin to implement the above logic:

function widget_context_currently_override( $currently ) {

	if( empty( $currently ) ) {
		return $currently;
	}

	if ( isset($currently['is_single']) && $currently['is_single'] && 'post' !== get_post_type() ) {
		// Ensure the widget doesn't display for other post types
		unset($currently['is_single']);
	}

	return $currently;
}
add_filter( 'widget_context_currently', 'widget_context_currently_override' );
New 'widget_context_visibility' filter

Provides even more flexibility so that other plugins could override any of the widget context visibility/display logic.

Default behaviour remains unchanged.
Merge remote-tracking branch 'upstream/master' into filter
Merge pull request #8 from om4james/filter

New filters so that other plugins can override widget context visibility checks