Refactor global options

Instead of having a global variable called `$options` (which is
obviously stupid, and poisonous if anyone includes Varnisher as
a library), set the options as an instance variable of the Varnisher
module.
Change option loading order

Previously, a user's RC file took priority even over command-line
arguments, which isn't sensible.

This commit refactors things so that the load order is:

1. Defaults
2. RC file
3. Command-line arguments
Set default options in one place only
Merge pull request #3 from robmiller/refactor-options

Refactor global $options