Erlang Strings

As described in #18 by @rgrinberg, with tweaks from @AndrewRadev:

autocmd FileType erlang let b:switch_custom_definitions =
      \ [
      \   {
      \     '<<"\([^"]*\)">>':       '"\1"',
      \     '\%(<<\)\@<!"\([^"]*\)"': '<<"\1">>',
      \   }
      \ ]

This switches between the forms "..." and <<"...">>.