Add formatters parameter to DataFrame.to_csv()

[adamboche]

DataFrame.to_string() has a formatters parameter for formatting columns individually, but to_csv() could benefit from that parameter as well. This is especially useful, for example, when formatting integers with missing values.

[jreback]

duplicate of #4668

yep, would be nice to have. interested in a PR?

[TomAugspurger]

Closing in favor of #4668

[moi90]

Actually, this is NOT a duplicate of #4668. There, float_format is talked about, but this applies to all columns. A formatters parameter, similar to to_string could apply different formats to different columns (and this is exactly what I need).

I request to reopen.

(I also just had a look at DataFrame.to_csv and it seems that is not that easy to rewrite to accept individual formatters for each column...)