Support of MainActivity.kt files

[maierhjakob]

As far as I understand it, trapeze currently only supports a MainActivity.java file cause within the getMainActivityFilename() function the .java extension is hardcoded.

Therefore it would be nice to have an additional optional parameter to specify the path to the MainActivity file with the extension used, e.g. MainActivity.kt

I could also implement a PR myself but I don't know if this is wanted or if you have any preferences on how this should be implemented.