Issue 374: Allow an option to choose the hashing algorithm

[Kpovoc]

• hashAlgorithm key added to DEFAULT_OPTIONS
• hashAlgorithm key can be overriden by value given in options argument
• buildOptions function will ensure a default a hashAlgorithm exists, and throw an error if the provided option is not supported by the system.
  • throws early error during app setup, rather than waiting until an upload is attempted
• memHandler and tempFileHandler use the hashAlgorithm option now
• Updated tests for buildOptions function in utilities.spec.js

ref: https://github.com/richardgirges/express-fileupload/issues/374