Add raw CAN tx support and .pcapng file loading.

This pull request includes:

1. Changes by Ethan Zonca (https://github.com/normaldotcom/cangaroo) that added Raw CAN tx support.
2. Changes by me to add support for loading .pcapng (Wireshark, etc) loading support.

I've just started playing with CAN and love your program! I have further changes in mind...

Please let me know what you think and if you are interested in pull requests or not.

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