Implement missing formulas

[Michał Kacprzak]

Hi I want to implement missing formulas from Airtable Documentation but I saw comment about need of rethink formulas module. Is rethink/redesing in progress?

[Alex L.]

Yes, I've got a branch going, but I haven't posted it for review yet. Will try to get that up soon. My general thoughts about it are:

• Formulas should be composable objects, so that a formula can contain other formulas
• We should be able to use operators like & and | to build complex logical conditions
• Objects representing formulas should only be rendered to str at the very end, right before the API call
• We should have a reliable way to represent every Airtable function correctly, without doing each one by hand

Whatever shape this takes, it will almost certainly be a breaking change from the current API.

[Alex L.]

See #329 for a proposed redesign.