Target & in .placeholder mixin

The previous approach added a descendant selector (space) which ramped
up specificity when not used globally. Since this mixin shouldn't
generally be used globally (.placeholder-styles becomes
non-overridable because of LESS's scoping rules), there was no way
around it. The new version operates on "&", meaning you can use any
selector to directly target the placeholder elements (input, textarea,
etc.) without adding unwanted specificity.