config: add SortKeys option to sort native map keys before display

If ConfigState.SortKeys is true, then dump and format will sort map keys
before displaying them. Only native types (bool, ints, uint, uintptr,
string) are supported, other slices are left unchanged.

The motivation is to have more diffable output, mostly for test purpose.
dump: make SortKeys default to string representation instead of nothing

The previous version of SortKeys was sorting only native types. Now, if
the type is unknown it defaults to its reflect.Value.String()
representation which at least guarantees display stability.
Merge pull request #13 from pmezard/add-option-to-sort-map-keys

config: add SortKeys option to sort native map keys before display