Auto merge of #4216 - durka:install-multi, r=alexcrichton

Auto merge of #4216 - durka:install-multi, r=alexcrichton

cargo install multiple crates

rust-lang-nursery/rustup.rs#986 for `cargo install`

Revives #2601 @pwoolcoc, replaces #3075 @esclear, closes #2585 @kindlychung @cyplo

Avoids the sticking point of the previous two PRs (multiple registry updates) by threading through a first-run boolean flag to decide whether `select_pkg` needs to call `source.update()`.

There is still the issue that flags such as `--git` and `--vers` are "global" to the multiple packages you may be installing. The workaround is just to run `cargo install` separately. In the future we could add syntax like `cargo install foo=1.0 bar=2.5 quux=git://github.com/durka/quux#dev-branch` or something.