Improve item recognition in "case … esac" statement (in particular, fix issue #77)

• Fix issue #77 (closed) (exclude variable assignment from item recognition)
• Allow quoted items to contain spaces

A detail: I don't know how to enable backreferences in regex (or if it is possible in this project), so I left this pattern as is:

'[^)]+'|"[^)]+"

With backreferences, this would be simpler:

(['"])[^)]+\1