Oh, I can just see it now too:  Users are going to want to be able to specify extensions by EXTNAME/EXTVER as well.  I think the 'ext' argument should be expanded to recognize this.  A common format is if given a non-integer string like 'SCI' then return the first 'SCI' extension (if it exists).  Else if given a comma-separated value like 'SCI,n' return the n-th SCI extension.

`HDUList` objects support exactly this kind of indexing, so it should be easy to add this in.  I already know *somebody* is going to ask for it :)