feat(PricingSlot): custom accessors

[Coriou]

PricingSlot has two new props: titleAccessor & priceAccessor, allowing to pass custom price & title components respectively

Not ready to be merged, but is working. Check the example:

$ npm start
# localhost:8080

To do:

• [x] Write / update tests
• [x] Document readme
• [x] Provide an example

[Coriou]

Spamming with PRs here. For a quick check of everything in #11, #10 & #9, you can have a look at coriou/react-pricing-table#develop

[gergely-nagy]

@Coriou thanks a ton for this pr and all the updates.

I will get to reviewing them today.

[Coriou]

Hey, have you had some time to take a look yet ? Thanks

[Coriou]

That was fast :)

I'll make myself available in Gitter if you need to discuss this or help with merging all the PRs

[gergely-nagy]

Thanks for your work! Trying to merge this PR ASAP 😄