Add Parsing Option that Specifies Class for Decimals

This commit addresses issue [#219 in flori/json] (https://github.com/flori/json/issues/219). A new `:decimal_class` option for the parser allows the caller to select an alternate object class when a decimal number is parsed, instead of the default `Float`. The new class should have a one-argument constructor and is passed the read decimal as a string.