Consider making commands be commands instead of flags

taskbook uses flags for all of the actions it can do:

```shell
$ tb --task hello world
$ tb --check 1
```

These commands do not compose as flags would imply in a CLI, so they should probably just be commands and not flags:

```shell
$ tb task hello world
$ tb check 1
```

This makes it a lot easier to type stuff out, and more coherent with how other CLI apps work.

No files, tests, or entry points are named. Start by locating the CLI argument parsing and existing command-related tests, then determine the scope of changing task and check from flags to commands and verify that the documented examples work with the new syntax.

Written by the indexing model from the issue text.