Add the ability to reuse insetter

[mercuriy94]

Hello!
I suggest adding the ability to reuse the liners. In some cases, this can be useful to get rid of code duplication.
Since the Insetter instance is immutable, we must reuse it.
For example:

            Insetter leftTopInsetter = Insetter.builder()
                    .applySystemWindowInsetsToPadding(Side.LEFT | Side.TOP)
                    .build();

            leftTopInsetter.applyToView(btnLeftTop1);
            leftTopInsetter.applyToView(btnLeftTop2);

            Insetter rightBottomInsetter = Insetter.builder()
                    .applySystemWindowInsetsToPadding(Side.RIGHT | Side.BOTTOM)
                    .build();

            rightBottomInsetter.applyToView(btnRightBottom1);
            rightBottomInsetter.applyToView(btnRightBottom2);

[chrisbanes]

LGTM, thanks!