Move the available configuration options to a separate toml file, and test it

Move the available configuration options in https://github.com/kellpossible/cargo-i18n#configuration to a separate toml file, and add a deserialization test for it to ensure that it doesn't get out of date.

Start with the configuration section linked in the issue and locate the code that represents and deserializes the available options. Move those options into a separate TOML file and add a deserialization test that detects when the documented configuration becomes outdated.

Written by the indexing model from the issue text.