Feature request: add ability to include headings before the TOC's `<!-- START -->` flag

[haysclark]

I've been converting some docs to MD that have TOC content before the TOC itself. e.g. ## Preface. Currently, only generates TOC content for elements found after the <!-- START -->. It would be nice if the was a doctoc flag would override this behavior and parse the entire document.

Naturally, this might require adding additional features to exclude referencing the TOC itself, like IGNORE flags.:

<!-- TOC-IGNORE-START -->
<!-- TOC-IGNORE-END -->

[fab1o]

Related question: Is there a way to ignore sections? For instance, I want to ingnore Section 3, I have this readme:

# Title

<!-- START doctoc -->
<!-- END doctoc -->

## Section 1

## Section 2

## Section 3

[thlorenz]

None of the above two is possible ATM, but I'm open to a PR that makes that configurable.
TOC-IGNORE-START is a bit too specific, but something along the lines of DOCTOC_INCLUDE_START and DOCTOC_INCLUDE_END and DOCTOC_SKIP_START/END comments could work.

Best to discuss details here with some more detailed examples before jumping into the implementation.

[AndrewSouthpaw]

Agreed. It seems like the --include-all would be easier to pull off than the one about skippable headers. Probably want to treat them as a separate features. :)