Carthage & Swift Package Manager support

[thebarndog]

Title says it all, I'll put a PR up

[thebarndog]

Hmm this is actually going to be more complicated than I realized. Because this was probably set up using the cocoapods gem, I think that carthage won't work because Notepad isn't a project, it's a scheme within Example.xcodeproj and even those it's marked as shared Carthage can't find it which is why I suspect it fails carthage build and carthage update commands.

I'll have to put up a larger PR or a series of PRs but there's a couple of things I want to change here:

1. Make the example project a standalone project and/or a Playground
2. convert Nodepad to it's own xcodeproj so carthage can find it
3. add fastlane for automation
4. add swiftlint for linting
5. manage gem dependencies via bundler
6. switch to git-flow
7. write unit tests

So really it's a lot of things but I'll probably do things in that order when I get a free afternoon.

[DivineDominion]

I see two options:

1. Doesn't Carthage work with Workspaces, too? Then you could add a root workspace to expose the shared scheme.
2. Create a Notepad.xcodeproj in the project root with 2 library targets (iOS and macOS). Share their schemes for Carthage, too. Then add Example-macOS and Example-iOS app targets to the project as well (which will be ignored by Carthage).

I'm no CocoaPods wiz, so I don't know if that will cause more trouble than good.

[thebarndog]

Yeah that's a better idea actually, great suggestions 👍

This repo desperately needs tests as well. No open-source library should be distributed without tests.

[ruddfawcett]

@startupthekid 😢you know I'm not good at tests!

[thebarndog]

Someday young padawon, someday you will be.

[ruddfawcett]

@startupthekid do you think you'll get a chance to work on this? No worries if not!