Instead of optionsMain exporting an async function, this commit changes
it to return a promise instead. We split all the needed async
helpers for this config builder into smaller promises, in `src/options/*`. Another side
effect of this is that we perform all our async config inferring in
parallel, which speeds up the nativefier CLI.
Add proper unit tests as well for all of these promises. Switch to
Jest for these unit tests, and we are temporarily running both Jest and
mocha together in `npm test`. To refactor all the Mocha code to use Jest in
a future commit.
