New functionality for zgrab2-banner: md5, sha1, sha256, base64

[crazylazyowl]

Hello! While using the banner module for zgrab2, there was a need for some functionality that was missing there.

I added the following:

1. calc md5, sha1 and sha256 of body
2. base64 encoding of an answer body
3. check for empty pattern

I also refactored the code a little.

Thank you!

p.s: I’m plannig to add similar functionality to the http module. Is it okay?