Customer placeholder chars using token expression.

Allows you to quickly define a customer placeholder using "{&lt;CHAR&gt;}" expressions that immediately follow valid mask definitions.

Example:

    // Setting mask to:
    mask = "9{M}9{M}/9{Y}9{Y}"; // Creates placeholder "MM/YY" instead of "__/__"

This patch does not affect existing functionality.

Edit: Fixing jshint error.