export is treated as unknown command

[nus-se-bot]

Describe the bug
Using export with any argument gives an error message "Unknown command".

To Reproduce
Steps to reproduce the behavior:

1. Type export /path/that/user/has/access/to. In my case, export /Users/Nerrons/photo-test.

Expected behavior
The current image file is saved at the given destination.

Screenshots

Additional context
Log:

Apr 05, 2019 2:55:57 PM seedu.address.logic.LogicManager execute
INFO: ----------------[USER COMMAND][export /Users/Nerrons/photo-test]
Apr 05, 2019 2:55:57 PM seedu.address.ui.MainWindow executeCommand
INFO: Invalid command: export /Users/Nerrons/photo-test

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Reported by: @nerrons
Severity: High

_{[original: nus-cs2103-AY1819S2/pe-dry-run#532]}

[ghost]

See #180