Prevent updating transient when just previewing post and prime cache upon post-save

[westonruter]

Currently the syntax-highlighted markup gets cached in a transient every time a modification is made, unless in development mode:

syntax-highlighting-code-block/syntax-highlighting-code-block.php

Lines 632 to 634 in 85bb16b

	if ( ! DEVELOPMENT_MODE ) {
	set_transient( $transient_key, compact( 'content', 'attributes' ), MONTH_IN_SECONDS );
	}

This can cause quite a few throw-away transients to build up in the course of editing a post. I suggest we add an is_preview() to the conditions, so it only caches the result when viewing the post in a non-preview context.

It would also be good to warm the cache upon save_post so that a frontend request is less prone to involve a cache miss.