Allow an option to choose the hashing algorithm

[Kpovoc]

Hi! First of all, thank you for all of your hard work! This module is super useful, and makes file-upload a breeze.

The Issue

My team is using this module in an app running in a restricted environment. FIPS compliant algorithms are strictly enforced, and this is causing an error when express-fileupload uses MD5 to hash the temporary files.

The Suggestion

I think a good solution would be adding a hashing-algorithm option to the setup, to be used instead of a hard-coded 'md5'.

I don't mind doing the work and putting in a PR, but I wanted to make an issue first to see if this solution was a desired one.

Thank you for your time!