Configuration file support

Whenever I run GitUI in a new working directory, when comparing differences, I have to do `o` to get into Options, then under Diff, I set "Ignore whitespaces" to `true`.

I would like it so that within the config directory, I can place a `gitui.ron` file with the content:

```ron
(
diff: (
ignore_whitespace: true,
context: 3,
interhunk_lines: 0,
)
)
```

Then when GitUI starts up, it uses these values as the new defaults, and any changes you make would be written to `.git/gitui` in the working directory.

Start by locating GitUI's startup configuration handling and the Diff options persistence path. Trace how defaults are loaded and how changes are written, then verify that values from a config-directory gitui.ron file apply at startup while working-directory changes are saved under .git/gitui.

Written by the indexing model from the issue text.