Preserve shortcodes of other plugins

[rhurling]

With this PR Lasso preserves most shortcodes of other plugins (I'm pretty sure it won't work with shortcodes inside html tags). See #107 for details

Sample Shortcode Plugin I used to make sure this PR works:

<?php
/*
Plugin Name: Current Time Shortcode
Author: Rouven Hurling
Version: 0.1
License: GPL
*/
function current_time_shortcode() {
    return date( 'H:i:s' );
}
add_shortcode('current_time', 'current_time_shortcode');

function hello_shortcode($attrs, $content = '') {
    return 'Hello, ' . $content;
}
add_shortcode('hello', 'hello_shortcode');

[rhurling]

Ah... This PR is probably against the wrong branch...

[michaelbeil]

PR against master is fine for now. We will take a closer look soon. Thanks much @rhurling!