Add export command: back up cache data to a portable file using pickle - #7

Summary

Adds support for exporting the cache contents to a portable file using pickle

Command added

• export FILE - takes a valid file path and export the cache content (key, value, expire, tag) to the file in bytes (pickle)

Notes

• Used format as pickle, since cached values are not guaranteed to be JSON safe types.
• Expiry conversion occurs at the export time. Cache's stored expire_time is an absolute timestamp, and set expects a relative time.
• Always overwrites the filepath given to the command, as provided by the user deliberately. (Further scope exists for confirmation)

Testing

• Added 2 tests, passing successfully. Test coverage is at 98.31%