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:

(
    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.