Template: Support & document dashes as valid characters in variable and property names

[michaelpb]

Allow '-' character in symbol names in template syntax. This is especially useful for props. Then, document + test one of the following paths:

1. Translate to camel when doing JS (keep JS free of properties with - -- probably best, since it's how loadFromDOM works)
2. Keep the same, and make sure - is used consistently (e.g. in script tags, props['my-best'] instead of props.myBest)