Change config loading to load .PS1 files

Change config loading to allow loading from @INC:
- if -p:
  1. try an AngelPS1::Prompt::$prompt module
  2. try a $prompt.PS1 (usually ~/.config/angel-PS1)
  3. die
- if -c:
  1. try the given file
  2. die
- else
  1. try an AngelPS1::Prompt::$name module
  2. try $name.PS1 from @INC
  3. AngelPS1::Prompt::Default