Auto merge of #2630 - alexcrichton:build-script-warnings, r=brson

Auto merge of #2630 - alexcrichton:build-script-warnings, r=brson

Add build script warnings, streaming output from build scripts to the console

These commits add a few features to Cargo:

* Cargo now recognizes the `warning` key in build scripts and will print warnings *after a build script has completed* if the build script is for a path dependency. That is, if a build script prints `cargo:warning=foo` then Cargo will forward that to the console if the crate's being developed.

* Cargo now accepts multiple `-v` flags for all commands. The `-vv` flag prints warnings for *all* upstream crates, not just the local ones.

* When the `-vv` flag is passed Cargo will stream the output of all build scripts to the console, allowing more easy debugging of what happened if the build fails later on.

More details can be found in each commit, and otherwise this

Closes #1106