Add `options.mode` to support new `always-ascending`

[MadLittleMods]

API discussion welcome 😀

options.mode

• additive: Adds the same amount of specificity to each selector (this functionality is what the plugin currently handles, see options.repeat
• always-ascending: Specificity that matches the source order. See http://philipwalton.com/articles/do-we-actually-need-specificity-in-css/

options.useAttributeIds: Whether to transform #foo into [id="foo"]. Somehow, maybe this should be attached to options.mode -> always-ascending as the article explains

cc @philipwalton

[philipwalton]

@MadLittleMods cool, I didn't realize this plugin existed!

A couple thoughts:

• I wonder if it's worth having the ascending-specificity idea discussed in my article be a separate module that depends on this module, perhaps postcss-ascending-specificity? Just a thought, obviously it's your call, but I'm afraid it might be confusing with so many options.

And then a few wish-list items:

• There should be an option to have a starting specificity, ideally it would be in a format like X.Y.Z, which you could use something like semvar to compare -- even though that's not really what it's used for :)
• Someone expressed a concern to me on twitter that [id="some-id"] is slower than #some-id. For people who care about that difference, you could add an option to set a root ID, allowing you to prepend something like #root#root:root:root:root to overcome ID specificity.

Also, I'd be happy to be cc'ed on a PR and discuss more there. Thanks for doing this!