Color in progress_message

### Description of the problem / feature request:

As a rule author, `progress_message` in `ctx.actions.run()` is one of my primary interfaces with a user of my rule. Perhaps `progress_message` is the primary interface for users of Bazel in general who don’t spend much time changing build configuration.

Currently, it appears like I can only write plain text for `progress_message` which isn’t very helpful if I’d like to draw user’s attention to the important parts of the message.

### Feature requests: what underlying problem are you trying to solve with this feature?

I’d like, specifically, to bold parts of the progress message that are important and dim parts that are less important.

For example in “Compiling Rust crate proc-macro2” I’d like to bold “proc-macro2” for “Compiling Rust crate **proc-macro2**”.

I’d like the coloring mechanism to hook into Bazel’s own color option so it can be disabled for shell’s that don’t support color.

Start by tracing how ctx.actions.run() handles progress_message and how Bazel's color option controls terminal output. Define how bold and dim formatting should be represented, how it behaves when color is disabled, and what tests demonstrate the requested behavior is complete.

Written by the indexing model from the issue text.