Allow certain non-numeric characters in the page number field

There are several classical books that may use a reference system that includes letters, e.g. a line in Aristotle or Plato maybe be referred to as 1492a. Other classical works may have a Book.Section.Paragraph, e.g. 9.4.48 A Bible passage would be "Gen. 1:1" Shakespeare is referred to by Act.Scene.Line such as (Ham. 1.2.15–20) Currently, anything other that [0-9] in the page number box is unable to be saved.