Configurable source mapping

We do some [cool stuff](https://github.com/bazelbuild/rules_closure/blob/master/closure/compiler/closure_js_binary.bzl#L50) to configure the `--source_map_location_mapping` flag. The only configuring it does at the moment, is getting rid of the weird bazel-specific directory names.

However we should also allow the user to specify his own path mappings. For example, he might want `java/com/google/foo/js/...` to be represented as `/assets/js/...` in the sourcemap file.

Therefore an argument should be added to `closure_js_binary` and `closure_js_deps` that accepts a string map of bazel directories to web server paths.