Run without any configuration when it is used immediately after installation

[ROCKTAKEY]

Thanks for your great package. I like this package so much that I create Emacs client for rhq.

This PR have 2 changes. These changes allow us to use full features of rhq without any configuration.

• config.toml becomes unnecessary to run
• Cache become unnecessary to run rhq list

config.toml becomes unnecessary to run

rhq needs config.toml, but it may be often annoying because rhq can be used without any configuration. For example, I use it without any configuration, so I just touch ~/.config/rhq/config.toml before use. On this PR, rhq just uses empty configuration when config.toml does not exist.

Cache becomes unnecessary to run rhq list

rhq list needs cache. However, the cache has repositories added only by rhq, so nonexistence of cache means no repositories are added, On this PR, rhq regards absence of cache as no repositories.