Create command line tool integration tests

[joshrotenberg]

The mdbook API has test coverage but the command line tool, which is probably more often used than the API, could probably benefit from some integration (and possibly unit) testing so that changes can be made with more confidence.

rustup has some examples. assert_cmd is also easy to use.

[bbros-dev]

Our 2c. From experience the more robust e2e test for a CLI is to actually run it as a user would. There are alternatives, but bats or some such setup is actually where you want to sink any time - in our experience.

[szabgab]

Some ideas for improvements:

• Check more details in the existing tests.
• Test with custom plugins.
• Maybe also test with existing 3rd party plugins.