adding --noSeparator override

Thank you for providing dynamo dump, it's great to backup/migrate tables between AWS zone and development life cycles.

In previous versions (prior to the separator) flag I was performing backups using '*' wildcard with a naming convention which didn't include prefixes

prodTable1
prodTable2
devTable1
devTable2
devTable3
stagingTable1

I could then do wildcard backups using tablename 'dev*' however this changed with the introduction of the separator.

I've added an override flag to the prefix separator to re-implement the wildcard functionality as it previously worked.

parser.add_argument("--noSeparator", action='store_true', help="Override flag the use of a prefix separator to use backup wildcard searches, [optional]")

I hope you might consider this pull request, it's semi-related to https://github.com/bchew/dynamodump/issues/6