Option to calculate the number of actions in execution phase

### Description of the problem / feature request / question:

Feature Request: Add an option to calculate the number of actions that will be performed in the execution phase.

Problem: The progress indication from Bazel does not give you a sense of the current progress because the number of actions typically increases throughout the execution phase.

It would be helpful to have a way to know how many actions will be performed so we can develop better UI/UX for progress which is less confusing for developers that work on a code base built with Bazel but are not familiar with it's internals.

### If possible, provide a minimal example to reproduce the problem:

Build any large project.

### Environment info
* Operating System:
macOS

* Bazel version (output of `bazel info release`):
0.5.3

### Have you found anything relevant by searching the web?
(e.g. [StackOverflow answers](http://stackoverflow.com/questions/tagged/bazel),
[GitHub issues](https://github.com/bazelbuild/bazel/issues),
email threads on the [`bazel-discuss`](https://groups.google.com/forum/#!forum/bazel-discuss) Google group)
No

### Anything else, information or logs or outputs that would be helpful?
(If they are large, please upload as attachment or provide link).

The issue provides no file, test, or entry point to follow. Reproduce it with a large Bazel build, then investigate how execution-phase progress is reported and how an option could expose the eventual action count. Done means the action total is available for improved progress UI without being continually increased during execution.

Written by the indexing model from the issue text.