Adds filter to get the credits link as string

[mlaetitia]

Changes proposed in this Pull Request

• New option return_output that basically allows the final site credits to be returned instead of echoed.
• If no value is passed the default behavior keeps being the current one: credits are echoed.

This is needed cause when the string is echoed automatically the only way to catch it to pass it through in a filter, for instance, is to use ob_start etc... However, this doesn't work IF the filter is called within an existing ob_start already which is the case I am working on

Testing instructions

• Checkout this and build the plugin
• Install it on a site you own
• Create a new page or post and add a shortcode block with the plugin's shortcode to confirm things still work
  • Bonus if you test the shortcode with the new arg to confirm nothing breaks (the new arg shouldn't be considered in the shortcode)
• Add the following to the site (or to a snippet using the code snippets plugin):

add_action( 'wp_footer', function(){ do_action('team51_credits', array() ); } );
add_action( 'wp_footer', function(){ echo '<br/>'; } );
add_action( 'wp_footer', function () { echo apply_filters('team51_credits_filter', '', array('pressable' => '') ); },14);

[georgestephanis]

Noting that I'm working with @mlaetitia in Slack on the use case, and whether we can get nested output buffering working.