Add (basic) autocompletion support #3

I know autocompletion would be quite hard, having to possibly parse .bzl files too (I don't think there's any API or command to list arguments given a rule or function, even less so a skylark rule or macro –there's Skydoc but it only generates documentation, though its code could probably be reused–), but at a minimum completing:

• rule and macro names (from the predefined list and the load() arguments in the file)
• target labels from the workspace (if there's a WORKSPACE file, aggregate all names from all BUILD files, or run a bazel query to gather them)

would be huge!

I understand that, as of now, this extension is “only” leveraging the Python syntax highlighting rules, tweaked for Bazel, and adding anything else would probably be a big effort, so only suggesting the improvement here.