add cf2list/2

this function allows you to group sections.

eg.

3> econfig:cfg2list(test).
[{"http \"private\"",[{"listen","192.168.1.2:8080"}]},
 {"http \"default\"",[{"listen","80"}]},
 {"http \"ssl\"",
  [{"listen","443"},
   {"ssl","on"},
   {"ssl_certficate","cert.pem"},
   {"ssl_certficate_key","cert.key"}]}]
4> econfig:cfg2list(test, " ").
[{"http",
  [{"\"private\"",[{"listen","192.168.1.2:8080"}]},
   {"\"default\"",[{"listen","80"}]},
   {"\"ssl\"",
    [{"listen","443"},
     {"ssl","on"},
     {"ssl_certficate","cert.pem"},
     {"ssl_certficate_key","cert.key"}]}]}]