Fix default configuration settings.

[inferiorhumanorgans]

• The current config file layout cannot be serialized to TOML with serde, so switch to YAML. This necessitates updating to an unreleased version of confy. See: alexcrichton/toml-rs#145 and alexcrichton/toml-rs#407.
• Print out the location of the default config file upon creation see: fixes #73, fixes #78.
• Set the default left prompt to something so the user isn't left scratching their head.
• Change the default icon set to Unicode so that this is usable out of the box with as few dependencies as possible.