Optional image / height props and documentation note

[Roilan]

@chromakode Updates from our discussion from #8

[chromakode]

Defaulting to 1px seems unwise to me; it's not an obvious behavior if you haven't used the module before and create an image with no dimensions. I would prefer that we require authors to specify an intentional width/height value (since this is email and sometimes CSS is not supported).

@Roilan, have you encountered a use case where specifying know dimensions is undesirable or not possible? I think it might make sense to be opinionated here: keep the dimensions requirements and document the use case (as you have in this PR).