Add support for transactions with payees

[pmarciniak]

This PR adds an option 'y'. If it's set, transaction entering flow will ask for a payee in addition to description. Entering payees has the same support for defaults and completion as entering descriptions.

Since either payee or description may now legitimately be empty, I've added a special value "-" to not accept the proposed default and enter an empty string instead.

Even with the 'y' option disabled, parsing of transactions is now aware of payees, so it no longer parses broken descriptions like Central Perk" "Scone.

[simon-v]

Why do you consider "descriptions" like Central Perk" "Scone broken? I've been using them to indicate payees for years.

[pmarciniak]

Yeah, I guess you can use them to enter new transactions, but when you want to search for a transaction with a given description/payee, it becomes a bit more messy. Also, some payees may have multiple descriptions associated with them, and vice versa (e.g. transactions with "Groceries" description may exist for multiple payees), which makes the auto-completion feature work in a suboptimal way.