Support multiple values format "<FIELD_NAME>[]"

[kale1d0code]

Thank you for the library,
amazing stuff,
especially the limiting functionality

I seem to have issues with multiselect and checkbox input types.
these sorts of inputs usually have multiple values (those with square brackets after the field name e.g. interests[])

currently pechkin only provides the last value instead of returning an array of values.

I haven't tried adding fields to my FormData object with the alternative format (
e.g.
interests[0]="foo"
interests[1]="bar"

however I was under the impression the first method was a HTTP standard. (seems to be default browser behaviour)

I looked at the code and I think adding a check for the square brackets for field names somewhere in FieldPromise.ts could support this edge case.