Feature request: add ingredient prices

[jayaddison]

Is your feature request related to a problem? Please describe.
Use case: as a person planning and preparing meals, I would like to be able to find out in advance the approximate price of the ingredients required to make each meal. This would be helpful for budgeting purposes and to select between different recipes.

Describe the solution you'd like
TBD - this could be a large and multi-stage task.

Describe alternatives you've considered
Scraping our own prices, looking for per-supermarket data feeds, and using crowdsourced pricing information (openculinary/api#54) could all be potential alternatives -- or perhaps complement bulk import.

Additional context
Could be related to #9 - we'll need a way to associate prices with products, and that could correspond to the way that we'll import and match metadata about products.

Potential data partners:

• Supermarkets themselves
• trolley.co.uk
• priceable.co.uk

Based on my own knowledge and background (I live in the UK) this is likely to have a bias towards the UK initially. That is something that would be good to design against -- in other words, as the interfaces and processes and data models are designed, they should be critiqued to check whether they are applicable, relevant, and feasible to implement for other countries.