Add toggle options to all features

[madeleineostoja]

No reason we shouldn't provide more fine-grain control over exactly what is included in a given Rucksack build, rather than just providing control over 'addons'. This is also becoming a standard amongst PostCSS frameworks (cssnano, cssnext, etc).

Syntax would be very simple:

rucksack({
  feature: boolean
});

Would have to either rethink how options are passed to CLI tool, or only allow control over addons there. Because a flag for each feature will get unwieldy.