get bower paths from .bowerrc [$15]

[austinpray]

Currently the bower path must be manually added as an option:

https://github.com/austinpray/asset-builder/blob/8f9570e9d4f8ea345c446a28e73234cfa8cd6a5b/test/test.js#L305

https://github.com/ck86/main-bower-files/blob/85b7102850645f9e6f42a27c81680f574da22476/lib/index.js#L32

https://github.com/roots/sage/issues/1362#issuecomment-76818140

--- There is a **[$15 open bounty](https://www.bountysource.com/issues/9130340-get-bower-paths-from-bowerrc?utm_campaign=plugin&utm_content=tracker%2F8853550&utm_medium=issues&utm_source=github)** on this issue. Add to the bounty at [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F8853550&utm_medium=issues&utm_source=github).