confgen: auto deduce whether the map key is unique

[wenchy]

By default, map key is not treated unique, in order to aggregate sub-field
(map or list) with cardinality. The map field key should be deduced to be
unique if nesting hierarchy is:

• map nesting map or list with different layout (vertical or horizontal).
• map nesting no map or list.
• map layout is incell.

[wenchy]

Currently, we can only support xlsx and csv formats. xml format should be later taken into consideration.

The PR #92 add BookFormat field to extInfo in SheetInfo, we can use it to distinguish importer format.