updating user-select, input-placeholder to use new CanIUse data

- adding support for ad-hoc CanIUse features
  - these files come from: https://github.com/Fyrd/caniuse/tree/master/features-json
- adding css-placeholder ad-hoc feature
- updating `user-select` and `input-placeholder` (in _user_interface.scss) mixins to use CanIUse data
- updating `inline-block`
- updating (in _regions.scss)
- updating `::selection`
- updating several other references to to `$legacy-support-for-ie6` and `$legacy-support-for-ie7`