feat: Add SplashScreen plugin

[jcesarmobile]

closes #133
closes #150

[carlpoole]

Couple of ideas

SplashScreenConfig and SplashScreenSettings could use Builders so the resulting object is immutable - it looks like those are only accessed inside SplashScreen and not set there.

the show method in SplashScreen might also benefit from a method signature without a Settings parameter so it just uses a default SplashScreenSettings. (similar to the old one that just took an activity like this:
public static void show(final Activity a) { show(a, DEFAULT_LAUNCH_SHOW_DURATION, DEFAULT_FADE_IN_DURATION, DEFAULT_FADE_OUT_DURATION, DEFAULT_AUTO_HIDE, null, null); })

[jcesarmobile]

As the idea of the implementation class is to make it easier to reuse the plugin by native code, it's easier to have a single object rather than a bunch of params.
Also, iOS had a TODO about the show having too many parameters, which wasn't good for the linter and the TODO comment suggested to use a struct, so I did. So on Android I used classes since there are no structs.

My code is based on Ian's approach on camera PR, which was approved by you and Dan, so if we want to use builders his PR should also be changed.

I removed the show with no params because it was not being used and iOS didn't have an equivalent, but I can put it back if you want. But then iOS should also have something like that for consistency.

[carlpoole]

I removed the show with no params because it was not being used and iOS didn't have an equivalent, but I can put it back if you want. But then iOS should also have something like that for consistency.

I'm thinking from a reusability standpoint for this method:

show(final AppCompatActivity activity, final SplashScreenSettings settings, final SplashListener splashListener)

You would call it:

show(activity, new SplashScreenSettings(), listener)

or you could also do

show(activity, listener)

if you just want defaults. If the first one is acceptable then that's fine.