Add skip_typescript attribute to prevent .d.ts emit

[joeywatts]

Fixes #1691 by adding support for a skip_typescript attribute which disables generation of type definitions for a particular Rust export.

[alexcrichton]

Thanks for this! I think though perhaps instead of typescript = false we could do something like skip_typescript? I'm not sure how many other things we'll want on the other side of the = for now perhaps?

It may also make sense to hold off on this for a bit while #2012 is sorted out, since that's a pretty unwieldy PR and may be hard to rebase over this.

[joeywatts]

skip_typescript seems good to me. If you prefer to wait on that PR, I don't mind rebasing/reworking this once that's in.

[alexcrichton]

Thanks!