Add --directory flag

[vmbrasseur]

#4 adds a --file flag for specifying the files to upload. This is a shortcut for that.

1. Add a --directory flag (not repeatable). May be used in conjunction with --file
2. Populate the files array by traversing this directory (recursively). All files in this directory will be added to the file array and will be uploaded
3. Do a check to confirm the directory is readable
4. Update documentation