Allow to specify multiple manifests

**Is your feature request related to a problem? Please describe.**
I have a Rust workspace containing both client and server code for our SaaS web app. For the client, we have several Rust packages that are compiled to WebAssembly and shipped with the client app, some for the main thread, some for the workers. So it would be helpful to be able to specify multiple `--manifest-path`.

**Describe the solution you'd like**
I would like to be able to tell cargo about which manifests to use (in my case all my client packages) and generate a single HTML output file with all dependencies combined.

**Describe alternatives you've considered**
I could include all server dependencies as well, it just makes the list too long. Or I could have several HTML output files, which is annoying. Or I could combine, manually or with a script, these files into a single HTML output file.

Start by tracing how cargo-about currently accepts --manifest-path and produces HTML output. Determine how multiple client manifests could be selected and how their dependencies could be combined into one output without including unrelated server dependencies. Done means the requested manifests produce a single correct HTML file.

Written by the indexing model from the issue text.