Adds support for a lens(ignore) attribute.

[jneem]

This isn't terribly important because #[len(name="_ignore")] is a decent work-around, but having an explicit ignore attribute feels cleaner to me. Also, I think factoring out the data attributes and the lens attributes is something we'll want eventually anyway.

[luleyleo]

I think these attributes should be documented! Feel free though to only document the one you added, the others can be added in a separate PR.

Also, the changlog entry is missing, code wise it looks good to me.

[luleyleo]

Looks great, thanks!