component-install Improvements

This PR includes 2 additional changes to component-install. (This is in addition to #320 (which fixes #305))

 * Download failures for remotes (other than the last) are now warnings instead of errors
 * Utilizing `Batch#concurrency()` via new cli flag `--concurrency <num>` to control `Package#getFiles()` concurrency rate.

The 3 changes in total have really helped me out with my team of developers. We are transitioning to Component.js, but still have a lot more dependencies than we should, causing random problems with component-install. (such as what I believe to be GitHub rate-limiting, giving `503` errors and corrupting installed components, and susequently the build)

These changes have done away with the problems we've had thusfar, so it would be really helpful to have them merged.