* Add support for Arrow IPC file format
Add support for Arrow IPC file format to dataframes commands. Support
opening of Arrow IPC-format files with extension '.arrow' or '.ipc' in
the open-df command. Add a 'to arrow' command to write a dataframe to
Arrow IPC format.
* Add unit test for open-df on Arrow
* Add -t flag to open-df command
Add a `--type`/`-t` flag to the `open-df` command, to explicitly specify
the type of file being used. Allowed values are the same at the set of
allowed file extensions.
