Make digit separator interval configurable

[christianparpart]

          This is fine for this PR, but in the future this should probably be configurable.

Also for defaults, 2 might be better for hexa and 8 might be better for binary, to group by byte. With octal we are doomed :P Maybe 4 because it is usually used for Unix permissions and those are 4?

Originally posted by @llucax in #4 (comment)