Add --num-format option to configuration file

[ian-h-chamberlain]

I tried configuring the number format in my ~/.config/tokei.toml, but it seemed to have no effect. Looking at the configuration struct https://github.com/XAMPPRocky/tokei/blob/master/src/config.rs#L23 it appears there is no corresponding option.

Would you accept a PR to plumb in the number format there to match other options that can be specified at the command line? Does the name num_format make sense for symmetry with other options?

Thanks!