Add `matches`, `querySelector`, `querySelectorAll` to DOM, and the `KEY_SVG_SELECTOR` transcoding hint

[carlosame]

Implement matches(), querySelector() and querySelectorAll()

• https://dom.spec.whatwg.org/#dom-element-matches
• https://dom.spec.whatwg.org/#dom-parentnode-queryselector
• https://dom.spec.whatwg.org/#dom-parentnode-queryselectorall

Add the KEY_SVG_SELECTOR transcoding hint

If KEY_SVG_SELECTOR is set and the document is HTML, its value as a CSS selector is used to locate the element that will be used as the SVG root.

In the case of the transcoding helper, if the selector argument is null then KEY_SVG_SELECTOR is used, and if also null then the previous behaviour follows.

If you come up with a better name for KEY_SVG_SELECTOR, please comment here.

This new hint is part of the effort to make EchoSVG friendly to embedded SVG images, see #40.