Added a function to select a sheet using a regular expression
Refactored out duplicate code when selecting a sheet either by name or by regex
Generalised select-sheet to find the first sheet which either returns true for an arbitrary predicate or whose name matches a string or regex
Merge pull request #18 from jonneale/select-sheet-regex

select-sheet can now select a sheet from a workbook using name, regex or predicate on sheet name