imp: Add ' as possible digit separator

[ThreeFx]

It is used for Swiss Francs: CHF 1'000

[crocodile-code-review[bot]]

Review on Crocodile

[Xitian9]

This requires careful consideration to make sure we don't mess up parsing in other cases. See related discussion at #1489, #273.

[simonmichael]

@ThreeFx, thanks for this. Some ideas to move it forward:

• update the docs in hledger.m4.md > Decimal marks, digit group marks
• add a test or two, I think numbers.test would be the right place
• at least manually sanity-check that it works as expected in the commodity directive and --commodity option