Automagically add unique id attributes to HTML headings

[caribpa]

Hi there!

This PR adds the option to automatically generate id attributes for the HTML headings by setting the :generate_heading_id option to true.

I am aware that #68 tries to do a similar thing, but it uses a <span> inside the heading instead of adding the id attribute to the heading itself, like this PR achieves.

The ids of subheadings are generated based on the ids of those above it, trying to preserve uniqueness.

In a nutshell, this:

* Hello
** World

Gets converted into:

<h1 id="hello">Hello</h1>
<h2 id="hello--world">World</h2>

This PR also adds the -i flag to the org-ruby tool to enable this behavior.

Let me know what you think 😁

[sangwoo-joh]

Thank you so much. Even though this commit has not been merged into the master branch yet, I personally use this feature gratefully.