Feature/config plugin

[ideopunk]

This adds a config plugin to make the SDK compatible with Expo. The readme has been updated with steps for Expo users.

[erzaehlsalex]

We have to integrate this sdk in our expo app as well! Does this also work with iOS? I have taken a look at your changes and it only includes changes for Android? Or am I wrong?

[ideopunk]

@erzaehlsalex When I ran expo prebuild, I found that my podfile platform was at v12, and that the android/build.gradle already contained mavenCentral(), so I decided to try just adding the multidex change to start. It worked fine on my ios and android, so here we are! I'd be curious to hear if it works for you.