Add 'complex tokens' to tokens.yaml

[nathankot]

This PR adds 'complex tokens' that look like this:

Expenses:
  Personal:
    - McDonalds
    - Pattern: /OFFSHORE SERVICE MARGINS/
      Account: Liabilities:Visa:Personal
  Business:
    Bank Fees:
      - Pattern: /OFFSHORE SERVICE MARGINS/
        Account: Liabilities:Visa:Business

The motivation being that sometimes transactions will have the same
description, but depending on the statement that they were recorded in,
need to go into different accounts.

It also adds a skip feature which I use like this:

Global:
  - Pattern: /PAYMENT RECEIVED THANK YOU/
    Account: Liabilities:Visa:Personal
    Skip: true

Things like credit card payments are usually recorded on two separate
statements (e.g Checking and Visa,) therefore it's desirable to skip one
of these transactions in order to prevent doubling them up.

Some more features could be added to the complex token in the future,
I'm thinking it'd be nice to define rules so that e.g a sum coming out
of a statement account can go into multiple accounts based on given proportions

[cantino]

Do you think it makes sense to move everything to complex tokens, adding these to complex_tokens as well, instead of having complex_tokens and regexps?

[cantino]

Thanks @nathankot!

[varac]

@nathankot, @cantino It would be lovely to finally merge this!

[nathankot]

@varac happy to rebase this if @cantino thinks this will likely go into reckon, personally I've moved to reading csv files with hledger instead