Support BigInt in the operators

[ilikejamesdw]

Bit odd that:

> numeral(10n)
{ _input: 10n, _value: 10 }

is fine. And,

> numeral(10n).add('10')
{ _input: 10n, _value: 20 }

is also fine. But,

> numeral(10n).add(10n)
Uncaught TypeError: Cannot mix BigInt and other types, use explicit conversions