title: Is there an event that is fired when HoT puts a cell into edit mode?
body: <p>I'd like to alter a cell's data, but only when it enters "edit" mode. There are a lot of events I can use, <code>onSelectionByProp</code> seems close but it's firing too often to be useful. Let's say, for instance, that I want to add '*' to a cell that has a value that is invalid in some way, but only when that cell is about to be edited. OK, it's a silly example but it's easier to explain that than what I'm actually doing. </p>
  
  <p>My current approach (haven't done it yet) is to find <code>TD.current</code> when a cell is double-clicked and then alter the text directly. Ideally I'd like to find a "retrieve data" event and alter what's coming back from that. </p>