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.