Adds option to count character length of placeholder

We often use the value of one field as a dynamic placeholder of another field if the second field, when empty, defaults to the first field.  This is probably a very common use of this feature.  However, sometimes those fields have different suggested maximums/minimums. We have been getting requests from editorial to display the "too long" text on a field that is inheriting text longer than its suggested maximum.

This request can be fulfilled by applying the same charactering-counting JS logic to an input's placeholder when its value is blank.  Sometimes, placeholders are used for things other than default text, so this behavior should be optional.  I made it an opt-in attribute of the ToolUi.Placeholder annotation.