This PR adds support for bazel WORKSPACE package files, as suggested https://github.com/alexeagle/angular-bazel-example/issues/17#issuecomment-349167982

Renovate will:
1. Detect `WORKSPACE` files anywhere in the repository
2. Look for all `git_repository()` sections in the file
3. Extract any dependencies with name, remote and tag values
4. Look up any dependencies that (a) have a github https remote, and (b) a valid semver as tag
5. Update the tag to the latest available