JVFloatLabeledTextView Can't adjust the placeholderLabel's x position

Hi,

I wanting to change the text inset of `JVFloatLabeledTextView`, everything is fine except the `placeholderLabel`. For content of the `UITextView`, I just need to adjust the `textContainerInset`, with `floatingLabel`, I can see there 2 public APIs is `floatingLabelXPadding` and `floatingLabelYPadding`. But with `placeholderLabel`, it has only one public API is `placeholderYPadding`.

So I'm wondering how to adjust the `placeholderLabel`'s x position?

Start at JVFloatLabeledTextView and inspect how placeholderLabel and placeholderYPadding are exposed and applied. Compare this with floatingLabelXPadding, then determine the smallest public API change that permits horizontal placeholder adjustment and verify the label position in the affected text-view behavior.

Written by the indexing model from the issue text.