Change type of program options to 'IO [Options]'

Some options have defaults that depend on the environment, and we
could handle these in two ways:

- use a place-holder value (e.g. data X a = Default | Custom a) that
  is later read from the environment
- move the options list to IO monad, where it can read the
  environment, etc.

The second option allows also displaying the actual defaults in the
`--help' output, even though it's not as nice, so I went with it.

This patch only changes the option types, without actually changing
any options yet.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>