IdAttributePlugin: append unique number sequence to duplicate IDs

[xplosionmind]

The IdAttributePlugin (#3363) throws an error if a page ends up having duplicate heading IDs.

As of right now, beyond changing the content of the headings themselves, the only option to avoid this error is to ignore it (adding the option checkDuplicates: false).

It would be absolutely fantastic if this plugin could append unique sequence of numbers after the nth duplicate heading, so that IDs would become unique.

For example, if I have the headings <h3>Example heading</h3> and <h2>Example heading</h2>, the plugin would generate and ID normally for the first one that appears in the page, <h3 id='example-heading'>Example heading</h3> and it would add -1 to the ID of the second heading: <h2 id='example-heading-1'>Example heading</h2>.

Related to #3424 and #3560.