Autogenerate html/svg known_tags from mozilla website information

[schloerke]

Fixes #84
Should fix alandipert/html2r#4

Automatically pull

• HTML elements from https://developer.mozilla.org/en-US/docs/Web/HTML/Element - all elements
• SVG elements from https://developer.mozilla.org/en-US/docs/Web/SVG/Element - only elements with documentation pages

Considered pulling information from these websites...

• W3 Schools
  • https://www.w3schools.com/tags/default.asp - Did not distinguish what should be considered obsolete
  • https://www.w3schools.com/graphics/svg_reference.asp - Had many extra tags such as altGlyph
• W3 Standard
  • https://www.w3.org/TR/2018/WD-html53-20181018/single-page.html - Could not distinguish what was not obsolete
  • https://svgwg.org/svg2-draft/single-page.html - Did not trust the resulting set that was web scraped

[wch]

A few things:

So that we don't forget what's going on here, the following would be helpful:

• At the top of the script, add an explanation of what it does.
• In tags.R, right above known_tags <- ...., add a comment saying how the content got there.

Have you checked to make sure we haven't lost any tags? It's possible that we had a weird tag that's no longer there.

[schloerke]

@wch

At the top of the script, add an explanation of what it does.

✅

In tags.R, right above known_tags <- ...., add a comment saying how the content got there.

Moved the generated tags to a file and gave it a roxygen2-like comment.

Now tags.R includes known_tags.R.

[schloerke]

@wch

Have you checked to make sure we haven't lost any tags? It's possible that we had a weird tag that's no longer there.

I pull in the tags from a CRAN install htmltools. This set is merged with the new set.

I also double checked the obsolete tags on SVG and had accidentally included them. They have been removed in 0bc3a62