Style position and length with commas

[dabreegster]

This is a fantastic crate, thanks so much for building and maintaining it!

I'm currently including {pos}/{len} in my template string, but for huge data, something like "33857009/42672348" isn't terribly readable to me. I'd like to style it as "33,857,009/42,672,348". I'm proposing:

1. Adding a HumanCount structure along the lines of HumanBytes
2. Adding human_pos and human_len as template keys, in https://github.com/mitsuhiko/indicatif/blob/0626585b006e9ba11df065c0d0d3df275d202257/src/style.rs#L270

Very open to better names than these. To actually do the formatting could be a bit tricky, since there are different styles based on locale: https://docs.microsoft.com/en-us/globalization/locale/number-formatting.

Option 1) Just hardcode the use of US-style commas.
2) Use num_format and allow the user to explicitly override a Locale, possibly as a new set_locale method on ProgressBar
3) Use num_format and enable the with-system-locale feature
4) Use num_format, let people specify the locale in the string (like {len:en}), and have some kind of parsing to get to the Locale enum

Any preferences here? I'm happy to send a PR; the hard part is making these decisions. Thanks!

[djc]

HumanCount, human_len and human_pos sound good to me. Let punt on the localization question and stick to US-style formatting for now. PR welcome!