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.

This is the same pull request than the one I did against origin:

  https://github.com/davecgh/go-spew/pull/13

The idea is I do not want to wait for its acceptance to use it, so I need to have this changeset in the masagroup fork, and in a durable way since it will be referenced by golibs submodules. So, I have created a masa branch in go-spew masa fork. *This branch should never be force updated*, otherwise it will lose references on past masa commits referenced by golibs. Instead, we should merge origin/master into it, possibly erasing former differences if all submissions have been accepted. I suggest we work the same way with other foreign repositories.

Comments on this and on the pull request?