fix elements selection

In browsers that support srcset, don’t exclude img elements that also
have a ‘sizes’ attribute, which isn’t supported for now. We should add
precise feature detection, really.

+ wondering how we can manage when `400w` is used in srcset but
`sizes=“100vw”` is implied but omitted (this isn’t supported by any
browser either afaik)
Merge pull request #184 from gBokiau/fix-elements-selection

fix elements selection