Use start= attribute of <ol> to enable numbered lists that start with non-1 digits

[jywarren]

(ref publiclab/PublicLab.Editor#128)

In GitHub Markdown, there's a nice feature where you can start a list with a non-1 digit and it starts numbering there. As a step in that direction, when parsing Markdown lists, can we not convert to 1,2,3,4 automatically? The megamark parser should be able to read it anyways, so if we take a list like:

3. hi
4. hi
5. hi

...is there any way to preserve that numbering while in rich mode? Perhaps by storing the number in a <li data-numeral="3"> type attribute?

Just by way of example:

3. hi
4. hi
5. hi

[jywarren]

Aha -- examining GitHub's work, the list starts with <ol start="3"> -- I imagine that's a standard HTML way to do this. Could that work?

[jywarren]

This line seems like where we could start:

woofmark/src/html/list.js

Line 12 in b58efb7

var olist = '<' + tag + '>';

and correspondingly on converting to Markdown, here we'd have to read the start attribute and offest the numerals shown:

woofmark/src/markdown/list.js

Lines 31 to 34 in 2d93211

	digital = /\d+[.]/.test(chunks.startTag);
	chunks.startTag = '';
	chunks.selection = chunks.selection.replace(/\n[ ]{4}/g, '\n');
	wrapping.unwrap(chunks);

[jywarren]

@bevacqua what do you think of this solution? Not urgent but if you +1 I can open a PR!