Progress indication for schematics

Since the `TSLintFixTask` can take a lot of time (e.g. if used to apply source code transformations for `ng-update`), it would be nice to have a progress indicator that shows that "something" actually runs.

For example: if someone runs `ng update @angular/material@next`, it's not clear for a period of time that the fix task actually "runs". See: https://github.com/angular/material2/issues/12774.

An option that can be specified when configuring the `TSlintFixTask` would be nice to have. The option would then show a simple progress indicator while TSLint runs.

---

Ideally there would be a way for a schematic to:

* Have an progress-spinner with a name of the schematic currently running
* Report a status (similar to how Bazel `repository_ctx` can report statuses)

Nowadays a lot of schematics need to run as part of an `ng update` and it would be helpful to know what schematic currently runs / what it currently does (if known)

cc. @jelbourn

Start by tracing the TSLintFixTask used during ng update and review how schematics currently report activity. Define the scope for a configurable progress indicator and the broader schematic status reporting described in the issue; done should make long-running fixes visibly active and identify the running schematic when possible.

由索引模型根据 Issue 内容生成。