Add action hook after getting template part

[andrezrv]

I think it should be nice to have the possibility to execute actions after getting a template part, not only before doing it, as it is now. I find this can be particularly useful to deal with output buffering, allowing to do things like this:

<?php
add_action( 'get_template_part_content', 'catch_template_part_output' );
/**
 * Start output buffering for template part.
 */
function catch_template_part_output( $slug, $name = null ) {
	if ( '' == $name ) {
		ob_start();
	}
}

add_action( 'after_get_template_part_content', 'release_template_part_output' );
/**
 * End output buffering for template part and print modified output.
 */
function release_template_part_output( $slug, $name = null ) {
	if ( '' == $name ) {
		$output = ob_get_contents();
		$output = '[Something before the original output ...]' . $output;
		$output .= '[Something after the original output ...]';
		ob_clean();
		echo $output;
	}
}

I'm thinking this particular method could give an extra option to theme developers who need to modify a very simple piece of output without having to create a new template or new functions to be embedded into an existing template. I know maybe some people won't consider this to be a recommended practice (I'm actually a little divided here), but you'd still have the possibility to trigger any other functionality you want on such hook.

[ocean90]

Related: #18561 for another use case

[SergeyBiryukov]

Related: #18803, #21506.

[wonderboymusic]

This should also have a filter at the top to bail from the function if you've determined that you can serve from the cache. Filters and actions need doc blocks

[iamfriendly]

Added a filter at the top of get_template_part() to bail if we wish to, add an action after locate_template

[iamfriendly]

Building on the work of @andrezrv I've added docs to the action and also, as suggested by @wonderboymusic added a filter at the top of the function to allow us to bail should we wish. Docs added to both the action and filter.