Add support for passing a custom target

[patrick91]

• ✨ Automatically open the browser when starting vibediff
• Add support for untracked files
• Add support for passing a custom diff target

[patrick91]

based on #6

[malvex]

@patrick91 what about passing it as the main argument, i.e. vibediff HEAD~1, without --target?

[patrick91]

@malvex done!