Support the `--directory` option

[deemp]

@maxonfjvipon

Problem

Currently, it's unclear where eoc looks for files when executing the commands phi, test, unphi, print.

Solution

1. For these commands, add the --directory option that accepts a path to a directory.
2. Make eoc look for files only in the specified directory.
3. By default (when the --directory option is not provided), eoc must look for files in the current directory.
4. The default --directory option value must be documented in the CLI.
5. eoc must report a human-readable error when it found no matching files.
6. eoc should never recurse into a .eoc directory unless this directory is explicitly specified via --directory .eoc.