* Simplify theme overrides
Theme overrides are now loaded the same way key overrides are loaded.
The config file, `theme.ron`, does not have to contain a complete theme
anymore. Instead, it is possible to specify only the values that are
supposed to override their corresponding default values.
* Document breaking change in changelog
* Test that override differs from default
* Convert existing theme to patch
This changes the config directory from the `dirs::cache_dir` to `dirs::config_dir`. It does however keep the cache directory as the default logging directory, as it seems a better fit.
It also adds a function, `migrate_config`, which is called at startup to move directory entries inside the "old" config directory to the "new" one (but it skips moving log files). The intention is that this function can be removed after a few releases when the likelihood of someone upgrading from 0.6.0 or earlier is fairly small.
Fixes#98
