Add {@any} helper.

- Must be contained in a {@select} block
- Renders its body if any truth test inside the {@select} block evaluates to true
- Can be included multiple times inside a {@select} block.

As part of this change, {@select} no longer clobbers the current context, so keys that begin with a dot work inside a {@select}. This closes #9, closes #42, closes #50, and closes #79.