Allow specifying a custom output directory

Allow specifying a custom output directory

This commit adds support to allow specifying a custom output directory to Cargo.
First, the `build.target-dir` configuration key is checked, and failing that the
`CARGO_TARGET_DIR` environment variable is checked, and failing that the root
package's directory joined with the directory name "target" is used.

There are a few caveats to switching target directories, however:

* If the target directory is in the current source tree, and the folder name is
  not called "target", then Cargo may walk the output directory when determining
  whether a tree is fresh.
* If the target directory is not called "target", then Cargo may look inside it
  currently for `Cargo.toml` files to learn about local packages.
* Concurrent usage of Cargo will still result in badness (#354), and this is now
  exascerbated because many Cargo projects can share the same output directory.
* The top-level crate is not cached for future compilations, so if a crate is
  built into directory `foo` and then that crate is later used as a dependency,
  it will be recompiled.

The naming limitations can be overcome in time, but for now it greatly
simplifies the crawling routines and shouldn't have much of a negative impact
other than some Cargo runtimes (which can in turn be negated by following the
"target" name convention).

Closes #482