Support for retrieving attributes just from context

[ZacSweers]

Would need another overload for Style, and remove the hard requirement for attributeset not to be null in the generated Barbershop#style(...) method.

private static final int[] ATTRS = new int[]{
        android.R.attr.listDivider
};

// Want to do this boilerplate
final TypedArray a = context.obtainStyledAttributes(ATTRS);
mDivider = a.getDrawable(0);
a.recycle();

// Probable method signature
style(Object target, int[] attrs);

[ZacSweers]

Closed since it kind of just falls under #19