feat(splashscreen): Add support for spinner on Android and iOS

[trancee]

This pull request will add support for displaying a spinner on top of the splash screen, both for Android and iOS.

There are some new configuration keys:

• showSpinner bool: if true will enable the spinner, defaults to false.
• spinnerStyle string: controls the style of the spinner.
  Android: horizontal, small, large, inverse, smallInverse, largeInverse.
  iOS < 13: whiteLarge, large, gray.
  iOS >= 13: large, medium.
• spinnerColor string: sets the color of the spinner, values are either #RGB or #ARGB.

Refers also to issue #928.

[jcesarmobile]

Added a comment.
And a few more things:

• Use 2 spaces instead of 4
• As most spinner names are different per platform, better use different config values for them, like androidSpinnerStyle and iosSpinnerStyle, otherwise will not work unless the user changes it every time.
• On iOS you have launchSpinnerStyle and launchSpinnerColor that are not mentioned on the PR explanation and not present on Android. Why is that?
• Don't add iOS 13 code yet, it's still on beta and might change. Also makes the tests to fail because that code is not available on Xcode 10.
• Document the new preferences in Splash docs

[jcesarmobile]

There is an UIColor extension already (UIColor.swift) with hex implementation, so move it all there. It's not good to have it inside another class, nor have hex duplicated.

[trancee]

Thank you for your feedback, @jcesarmobile.

Please have a look at the changes, I hope I made it all right this time. Let me know if you have any other suggestions.

[trancee]

@jcesarmobile I am using swift-format as suggested in this blog post:
https://nshipster.com/swift-format/#swift-format

This is the only one that uses 2 spaces instead of 4. Even if do copy/paste of code in Xcode itself, it will change to 4 spaces. I am not sure what kind of code formatter you are using, but I am open for any suggestion to do it the proper way.

[jcesarmobile]

I don't use any formatter.
You can configure Xcode to use 2 spaces instead of 4 globally, or set it per file before editing it (probably this is better for you as you probably use 4 in other places)
See screenshot showing both the global preference and the per file on the right side

[trancee]

@jcesarmobile Thank you for the suggestion, it works that way in Xcode with 2 spaces indentation.

[jcesarmobile]

I've made a few code changes on the iOS code fixing some mistakes and code formatting.

Also simplified the iOS styles to just large and small, making large the default to match android default, because as they can be colored I think it doesn't make sense to have a grey option that you can change to another color.

Android code looks good, going to merge, thanks!