Extract config parameters to a separate file instead of using local.properties

[artem-zinnatullin]

Since 2.x will already be breaking #19, we can also extract mainframer config parameters to a separate file since Android Studio adds not very funny header to local.properties (though I've never saw such behavior):

## This file is automatically generated by Android Studio.
# Do not modify this file -- YOUR CHANGES WILL BE ERASED!

Something like .mainframerconfig should be fine I think.

[ghost]

@artem-zinnatullin the config file option should be probably updated in the README file

[artem-zinnatullin]

Sure, we'll update README before v2.0.0 release, it's still in development at the moment, created #72 to track this.