Add `lto` option to profiles.

Closes #759.
Merge branch 'lto' of https://github.com/BurntSushi/cargo into BurntSushi-lto
Polish off the LTO branch
auto merge of #895 : alexcrichton/cargo/BurntSushi-lto, r=brson

Closes #759, continuation of #790.
Merge remote-tracking branch 'upstream/master'