Slideshow: add filter to customize speed of the Slideshow

[jeherve]

This filter change should allow site owners to change the speed of all slideshows on their site, like so:

function jeherve_fast_slideshow( $args ) {
    $new_speed = array(
        'speed' => '10'
    );
    return array_replace( $args, $new_speed );
}
add_filter( 'jetpack_js_slideshow_settings', 'jeherve_fast_slideshow' );

Suggested in https://wordpress.org/support/topic/gallery-slideshow-settings-editable-somehow

[samhotchkiss]

Moved out of the pipeline until it's completed

[eliorivero]

@jeherve is there anything else you need to add? This is working fine on my tests and it just needs a rebase before merging.

[jeherve]

It seems to work well in my tests as well. I rebased. This should be read for merge.