Add `pym_id` attribute to the shortcode to allow custom ID names

This grows out of #20:

> We should probably make a new `pym_id` parameter on the shortcode that allows end users to configure the ID of the generated embed on the page.

https://github.com/INN/pym-shortcode/blob/master/pym-shortcode.php#L54 would change to something like:

```php
( isset( $atts['pym_id'] ) ) ? $atts['pym_id'] : "pym_$pym_id" ,
```