Allow dependency-path default value to be specified in environment variables.

[cprussin]

According to the bower docs you should be able to specify the components path using the environment variable bower_directory. But that environment variable isn't supported in the pulp implementation here, forcing pulp consumers to either pass the option as a CLI flag or create a .bowerrc to specify it.

The use case where it would be nice to read off the environment is when using a tool like bower2nix to do builds. Then the shell.nix can just export bower_directory and you have a valid environment, without having to symlink in the bower_components directory, manage mutating.bowerrc file, or modify the Makefile/package.json scripts to be nix-specific.