Process named ranges

[alberthat]

Implementation of report on named ranges per issue #317

Consistently with the discussion under issue #317, this adds a function excel_names which returns a tibble with three columns:

• name = defined name
• context = sheet on which name is defined (NA for workbook level name)
• ref = range within workbook, for names which define a local range

This has been implemented for both xls and xlsx formats. Implementation for xls required adding to the libxls code for this project, to record the relevant data structures for constructing the defined names and formulae.

I have included documentation, as well as unit tests in: tests/test-read-names.R