* Add support for Option<T> attributes
Introduces the `attr=[value]` syntax that assumes `value` is an
`Option<T>`. Renders `attr="value"` for `Some(value)` and entirely
omits the attribute for `None`.
Implements and therefore closes#283.
* Call `Generator::splice` directly
* Handle struct literals (edge case)
Co-authored-by: Chris Wong <xbuns@google.com>
