[subset] Err if options are not provided for list values
[subset] Allow pyftsubset --layout-features? etc

To see the current value of an option, pass a value of '?' to it, with
or without a '='.
Examples:
  $ pyftsubset --glyph-names?
  Current setting for 'glyph-names' is: False
  $ ./pyftsubset --name-IDs=?
  Current setting for 'name-IDs' is: [1, 2]
  $ ./pyftsubset --hinting? --no-hinting --hinting?
  Current setting for 'hinting' is: True
  Current setting for 'hinting' is: False

Fixes https://github.com/behdad/fonttools/issues/142