Add an ability to change the name of the generated object

[yshrsmz]

to implement #31, we first need to add an option to change the name the generated object.

API could be

buildkonfig {
    packageName = 'com.example.app'
    objectName = 'AwesomeConfig'  // defaults to 'BuildKonfig'

    defaultConfigs {
        buildConfigField 'STRING', 'name', 'value'
    }
}

🤔