Tests

- [ ] `parser.rs`: the parser should be property tested with something like `quickcheck`
- [ ] `cmd.rs`: `CommandLine` should be tested with ascii and utf8
- [ ] `resources.rs`: Gif export should be tested
- [ ] `session.rs`: inputs (mouse, keyboard) should be fuzz-tested to ensure nothing can crash the program.
- [ ] `session.rs`: `selection/*` commands should be tested in headless mode
- [ ] `session.rs`: all commands that take user input should be tested with unexpected inputs to ensure there is no division by zero for example
- [ ] `lib.rs`: rx's entry point should be tested with different inputs
- [ ] Test that everything that modifies a view sets the `[modified]` flag on it