Add support for localeOptions option: allow customization of i18n output

[christopherscott]

This PR adds support for passing an additional parameter localeOptions that is used when an explicit locale is passed to allow for further customization of the output. Defaults to {}.

This is particularly useful for places where filesize is highly dynamic/animated and there is a desire to enforce a number of fixed digits, even in cases where the decimal would be zero:

Example without (notice the left-shift of the digits when no zero decimal):

Example with:

Usage:

filesize(1024, {
  locale: "en-us",
  localeOptions: { minimumFractionDigits: 1 }
})

// => "1.0 KB"

The options are from ECMA-402, and are documented on MDN.

[avoidwork]

Thanks! I'll prep a release