Support binding from custom classes

[SUPERCILEX]

@JakeWharton elegantly fixes #43, fixes #10, fixes #6, fixes #42, fixes #9

All other previous solutions require extra work from the client whereas this solution uses a somewhat naughty hack to make life easy. Example use:

class AuthHelper(activity: Activity) {
    private val rootView: View by activity.bindView(R.id.root)
}

This implementation stores a strong reference to the context (which is fine since bindView should only be used in classes properly tied to the lifecycle anyway) by keeping the extension function's reference. In previous versions, the extension type was only used to not overload the different bindView methods, but we now use that context.

[SUPERCILEX]

I'm using JetBrains' Kotlin extensions now. It's disappointing to have never received a response. 🤷‍♂️