add an easy way to load flavor from arbitrary properties file

[yshrsmz]

def configureBuildKonfigFlavorFromLocalProperties() {
    if (project.gradle.startParameter.projectProperties.containsKey("buildkonfig.flavor")) {
        // prefer cli parameter
        return
    }
    if (Files.exists(Paths.get("$project.rootDir/local.properties"))) {
        def key = "buildkonfig.flavor"
        // load buildkonfig.flavor if exists
        def localProperties = new Properties()
        localProperties.load(new FileInputStream("$project.rootDir/local.properties"))
        if (localProperties.containsKey(key)) {
            project.setProperty(key, localProperties.getProperty(key))
        }
    }
}

personally I wrote this script and use it in my project. But it would be nice if this is baked into BuildKonfig

[yshrsmz]

@jfrey-irhythm Take a look at #128