Add `handleUploadedFileCallback`

Can be used to process file - extract images from video for instance - or upload to alternative storage methods.

Closes #96.
Closes #154
Closes #171
Add initial `nameCallback` support for handling file naming. Currently only handles the original filename

This can be used to rename the file appropriately on upload. For instance, if a file has accents, it may be useful to strip those out. You can also use this to hack around duplicative file names.

Refs #2
Refs #33
Refs #118
Refs #143
Refs #191
Closes #219
Closes #89