Twenty Fifteen: Add theme support for title tags

[obenland]

r30074 introduced a first step in the direction of creating a better experience around title tags for users.
It lets themes opt into letting WordPress handle the title output, to guarantee consistency between themes, and eventually enable plugins to have full control of the entire title output.

Let's lead the way, with Twenty Fifteen.

[iandstewart]

In 30097:

Twenty Fifteen: leading the way with support for title-tag

Props obenland, fixes #30146.