Product lookup by slug

[michaelbromley]

Is your feature request related to a problem? Please describe.
There is no way to lookup a product by its slug. The purpose of the slug is to be used in URLs, but without a slug-based lookup, the ID would also need to be stored in the URL too (as is the case with the demo storefront currently).

Describe the solution you'd like
First of all, slug values must be guaranteed to be unique (see #103). Then a lookup can be done either by extending the current product query so that the first argument is idOrSlug, or create a new query productBySlug.

Todo: research the pros & cons of each of the above alternatives.