Add ProGuard / R8 config

[JBramauer]

In my project, I'm using ProGuard. Enabling it without any CoreMidi4J specific rules crashed my app on MacOS (I'm guessing it's the native library accesses Java classes).

Adding keep rules for everything CoreMidi4J related fixed this for me:

-keep class uk.co.xfactorylibrarians.coremidi4j.** { *; }
-keepnames class uk.co.xfactorylibrarians.coremidi4j.** { *; }

This might be more than required, but in this case I took the safe route.

A bug in another library forces me to have optimization disabled as well.
CoreMidi4J might not need this, but I can't verify atm so if anyone else stumbles across this and still has issues after adding above rules, try this one as well:
-dontoptimize

In general, it would be nice to have rules for ProGuard/R8 embedded via META-INF.

[brunchboy]

This is open source. We have no desire to support obfuscation. Indeed, please be cognizant of your responsibilities under the license to make the un-obfuscated source code available.

[JBramauer]

Even if you don't want to actively support obfuscation, it is imho good practice to have the library not crash if it is used in a project with ProGuard enabled. I will, of course, obey the license if I indeed end up forking it for the library loading.