Add an --sexp option to "opam config env"

This patch adds an "--sexp" option to "opam config env" that prints the new environment as an s-expression. This is useful for allowing things like emacs to read the new environment.

Example output:
```
$ opam config env --sexp
(
  ("CAML_LD_LIBRARY_PATH" "/home/leo/.opam/4.00.1/lib/stublibs")
  ("OCAML_TOPLEVEL_PATH" "/home/leo/.opam/4.00.1/lib/toplevel")
  ("MANPATH" ":/home/leo/.opam/4.00.1/man")
  ("PATH" "/home/leo/.opam/4.00.1/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/usr/NX/bin")
)
```