Enchant `NumeralSystem` to accept complex system such as Roman numeral system.

[neizod]

so this:

NumeralSystem({ 'I':    1, 'IV':   4, 'V':   5, 'IX':   9,
                'X':   10, 'XL':  40, 'L':  50, 'XC':  90,
                'C':  100, 'CD': 400, 'D': 500, 'CM': 900,
                'M': 1000 })

will returns correct Roman numeral system.

implementation details

• key-value of input dict may be swap.
• parse int -> str should be very easy (due to the rule)
• parse str -> int require lexical (regex) analysis.