add hashAlgorithm to options; move hash to object instance property

[buzcarter]

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[buzcarter]

A security pal pointed out that sha1 has myriad security concerns, was asked to use a different algorithm.

Fortunately, it's an easy task to extend the initialization options to include a hashAlgorithm property (that we default to "sha1" for backwards compatibility)

Node crypto supports anything available in its native OS : https://nodejs.org/api/crypto.html#crypto_crypto_createhash_algorithm_options

The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256', 'sha512', etc.

[buzcarter]

this is the biggest change: moving this method from a private "helper" to a public class method, all necessitated by needing access to this.hashAlgorithm.

Is the naming convention to prepend an underscore to indicate private? (which would be great, IMHO -- _hash)