Support string quoting with `

[Gallaecio]

I’ve seen strings quoted with `, and web browser consoles seem to interpret them as valid quoting characters for strings, but js2xml considers them invalid characters.

For example:

    var a = `asdf`;

[Gallaecio]

It looks like this is called “template literals”. We should make sure this works once https://github.com/calmjs/calmjs.parse/pull/15 is merged upstream.