Add optional config file sections, selected with CHOOSE

Add optional config file sections, selected with CHOOSE

Allows the user to define arbitrarily named optional sections
in the config file, and select which of those sections are used.

For example:

  OPTIONS=color
  CHOOSE=char A,char B

  [char A]
  OPTIONS=role:arc,race:dwa,align:law,gender:fem

  [char B]
  OPTIONS=role:wiz,race:elf,align:cha,gender:mal