add support '0x' '0b' '0o' strings

[bdtren]

This is for fixing issue https://github.com/peterolson/BigInteger.js/issues/205#issue-602199428 My approach is convert src data(whenever it is number or string), compare it with prefixes( '0x' '0b' '0o') And use built in function parseInt() to convert it.

[peterolson]

A few issues with this:

• parseInt only works for numbers small enough to be represented by a native Javascript number, so this does not support larger numbers, which defeats the point of a big integer library.
• This could potentially be a breaking change for parsing strings that happen to start with 0x/0b/0o in other bases. For example, bigInt("0xfabb98", 36).
• Support for hex/octal/binary along with any other integer base already exists with the second argument, so these magic strings at the beginning are redundant.