Allow Config.new to take a hash of arguments and ignore defaults

Added a :without_defaults option to Sauce::Config.new.

Usage:
```ruby
c = Sauce::Config.new(:port => 111, :without_defaults => true)
c[:port]  #= 111
c[:host]  #= nil

c = Sauce::Config.new(:port => 111)
c[:port]  #= 111
c[:host]  #= 'ondemand.saucelabs.com'
```

Passing false will still cause an empty, undefaulted config to be
returned.