Feature request: watching updates docs

Great tool btw.

But I was using the `doc-open` command (by pressing D) to build and open the documents (presumably using `cargo doc --open`) and noticed that when source files are changed the docs are not rebuilt.

This feature to trigger a rebuild and to refresh the browser window (if possible) would be really useful when dealing with documentation comments. For example, you can hit `D` on bacon, add comments throughout your source code, and refresh or view in the browser the changes immediately.

Start with bacon's `doc-open` command, triggered by pressing `D`, and check how it invokes `cargo doc --open`. Reproduce the behavior by changing documentation comments while bacon is running. Done means source changes trigger a documentation rebuild and refresh the browser window when possible.

Written by the indexing model from the issue text.