Variadic definitions for `intersection`

@gcanti I'm not sure you went that path of not but that may be usable to generalise 'intersection' signature: https://github.com/gcanti/io-ts/blob/master/src/index.ts#L1191

I've tried some modifications but it seems to fail with previous typescript versions. Doing a PR in order to iterate/discuss on this.

Branch 312 https://github.com/gcanti/io-ts/tree/312