Add Buffer and KeyObject as possible key types

[davidje13]

Fixes #97

This adds Buffer and KeyObject as possible parameter types for key.

decode gets multiple overloads to detect when the branch

node-jwt-simple/lib/jwt.js

Line 79 in c58bfe5

if (!algorithm && /BEGIN( RSA)? PUBLIC KEY/.test(key.toString())) {

is triggered (if it is triggered, the key must be a string or Buffer; otherwise it can also be a KeyObject)

If that check were updated to be KeyObject-aware (see KeyObject.asymmetricKeyType), the decode types could be simplified to a single declaration:

export function decode(token: string, key: KeyLike, noVerify?: boolean, algorithm?: TAlgorithm): any;