basic SASL DIGEST-MD5 support

[nelapsi13]

added very basic support for SASL DIGEST-MD5 bind
basically, bind flow should be smth like

1. run bind_sasl_digest_md5_step1()
2. parse SASL challenge from step1 (at least nonce is needed)
3. generate SASL challenge_response (there is an example in lib/messages/sasl_digest_md5_bind_request.js )
4. run bind_sasl_digest_md5_step1(challenge_response)

[jsumners]

1. Can you add some unit tests for this new functionality?
2. Please update the documentation: https://github.com/ldapjs/node-ldapjs/tree/ad451edc18d7768c3ddee1a1dd472d2bbafdae5e/docs

Also, please rebase onto the next branch.

[sand123]

Any chances to see this functionality? I've stuck in SASL bind requirements

[jsumners]

If you would like to open a new pull request to add this functionality, you are welcome do so. Remember to add unit tests.

[binhphi109]

Are there any updates regarding this feature, or any plan?