Special-case svg tags, preserve case and slashes

Fixes https://github.com/kangax/html-minifier/issues/285

This is a pretty heavy-handed approach in that we override the user-supplied `keepClosingSlash` and `caseSensitive` options when the minifier encounters an svg tag.

Options are stored in a stack, and creating the stack relies on `Object.create` which may not be supported in all our runtimes.

Imbalanced input can lead to an over-popped options stack and a very ungraceful failure.