Add base10 support

Not everyone nor every use case can depend on base58 for generated ids,
so I am adding support for built in `bignum.toString(10)` which will
return a string representation of the flake in base 10.

`bignum.toString(16)` is irrelevant as it is already supported as
`Buffer.toString('hex')`.
Missing README.md updates
Merge pull request #1 from erickrdch/base10

Add base10 support