4.2: Add shortcode `[rwmb_meta meta_key="..."]`.

• Bug fix: save only last element of select field with multiple values. Link
• Improvement: add js_options attribute for date, datetime, time fields to adjust jQuery date/datetime picker options. See demo/demo.php for usage
• Improvement: add options attribute for wysiwyg. You now can pass arguments same as for wp_editor function
• Improvement: clone feature now works with checkbox_list and select with multiple values
• Improvement: add rwmb-{$field_type}-wrapper class to field markup
• Improvement: Add [rwmb_meta meta_key="..."] shortcode. Attributes are the same as rwmb_meta function.
• Code refactored