refactor sec.js

[pkra]

While many section-like elements are handled by sec.js:

texml-to-html/lib/transformer.js

Lines 107 to 109 in d7244f6

	// sec-like elements
	['ack', 'front-matter-part', 'dedication', 'book-app-group', 'book-app'].forEach(
	mapTag.bind(null, this['sec'])

some are not (e.g., abstract, app, notes) which is inconsistent. Also, some logic is duplicated (e.g., Ack handling in sec.js and app.js).

We could split up sec or we could let it handle these other elements, too.

[pkra]

Given how relative messy sec.js already is and how positive the experience of #452 was, I'm leaning towards the former.

[pkra]

Note to self: I don't think I'd do the heuristic for doc-introduction again today. I find the role questionable nowadays but "luckily" it doesn't do anything in real life. (If AT did, say, announce the role, then our approach shows how bad that would be - it would inevitably lead to lots of duplication.)