Allow to customize HTML output

First of all, thank you for making this great plugin.

I faced some issues when I tried to implement the following things on my website:

1. I have multi-language website with Polylang so I need TOC title to be localized. Hovewer there is no direct way to do that. Obviously implementing support for various multi-language plugins would be difficult and time consuming. Which is why I suggest adding a filter: $toc_title = apply_filters('toc_heading_text', $toc_title);

Right now in order to solve this I have to use option_toc-options and default_toc-options filters in order to modify plugin settings depending on the language.

2. I feel like wrapping the list <ul class="toc_list"> in the <nav> tag would be a good idea. Then again, in HTML5 there is rarely a “proper way” when it comes to tags. In any case some way to modify the output of the plugin would be nice. It could either be a simple $html = apply_filters('toc_result', $html); or a more complex system where your plugin uses templates that can be overridden by a child theme.