Auto merge of #2335 - fpgaminer:config-target, r=alexcrichton

Fixed #2332.

This PR adds `build.target` to the Cargo config file, which behaves in the same way as passing `--target` to Cargo.  Example `.cargo/config`:

```
[build]
target = "thumbv6m-none-eabi"
```

Similar to how `--jobs` overrides `build.jobs`, `--target` will override `build.target`.

I added documentation to `config.md`, and a test to `test_cargo_cross_compile.rs`.  I couldn't get cross compile working on my machine for `cargo test`.  Hopefully travis passes it.

This is my first PR against Cargo; sorry if I missed any procedures.