`select` should allow skipping non-existent files

https://github.com/openaps/openaps-contrib/blob/master/openapscontrib/timezones/__init__.py#L295-L296

Add some kind of `add_argument('-S', '--strict', action='store_true', default=False, help="..."), and change line 296 and thereabouts to call`os.path.exists` before attempting to read the file.
