Use "substring match" rather than "starts with" selector to match responsive units.

[msweeney]

Apologies for not catching this sooner, but the responsive units matching:

.pure-g-r > [class ^= "pure-u"] {
    width: 100%;
}  

fails when another class precedes the unit class.

This PR updates the selector to the following:

.pure-g-r > .pure-u,
.pure-g-r > [class *= "pure-u-"] {
    width: 100%;
}

I've added the trailing hyphen to avoid matching on pure-user or other potential classes that may start with pure-u.

One caveat is that this will also match classes that include the string pure-u-, for example impure-u-*.

[tilomitra]

@msweeney Could you update the README.md to add this feature as a bullet under "NEXT"? I'll pull this in after that.

[ericf]

Could you update the README.md to add this feature as a bullet under "NEXT"?

s/README/HISTORY/