adding support for "." as a thousand separator

[emarondan]

• Added new argument to fromstring to determine manually if "." is a thousand separator.
• Added logic to fromstring to check if a currency needs to have a "." as a thousand separator using CURRENCIES constant.
  Can fix "." incorrectly interpreted as a thousands separator #46

[Gallaecio]

Looks good to me, and I think we can ignore the 2 failing jobs (3.6 should be dropped, probably along with 3.7, and the mypy issue seems like something a new mypy should not complain about given the if ensures digit_group_separator is not None).

[wRAR]

3.6 should be dropped, probably along with 3.7

I've created #60 for this after seeing the failures here.