Add thousands separator to large numbers

[mundschenk-at]

From @mundschenk-at on January 11, 2016 14:2

See http://typefacts.com/artikel/zahlengliederung-digital for an example.

Copied from original issue: mundschenk-at/wp-typography#21

[mundschenk-at]

Also requested in https://wordpress.org/support/topic/feature-request-automatic-thousands-separator-with-safe-exclusions/.