Support aux dates

[tbm]

Support aux dates from (configurable) meta-date:

2018-03-17 * "Test with transaction aux date"
  aux-date: 2018-03-16
  Assets:Test                        10.00 EUR
  Equity:Opening-Balance            -10.00 EUR

[tbm]

2018-03-18 * "Test with posting aux date"
  Assets:Test                        10.00 EUR
    aux-date: 2018-03-13
  Equity:Opening-Balance

2018-03-18 * "Test with transaction and posting aux date"
  aux-date: 2018-03-17
  Assets:Test                        10.00 EUR
    aux-date: 2018-03-13
  Equity:Opening-Balance

[tbm]

and posting dates:

2019-01-16 * "Posting date using comment"
  Assets:Test                        10.00 EUR
    date: 2017-03-04
  Equity:Opening-Balance            -10.00 EUR
    date: 2017-07-08

[tbm]

Fixed in commit da32a4c