Feature request: Allow setting (multiple) targets from the command line

[jrose-signal]

Is your feature request related to a problem? Please describe.
We compile for several different platforms, each of which uses a different set of targets. Ideally the iOS dependencies wouldn't show up in the Android builds and vice versa, but today the only way to do that would be to have two about.toml files, which would mean duplicating all our clarifications.

Describe the solution you'd like
Add --target to the command line, which overrides any targets setting in about.toml. (This is consistent with Cargo command line options overriding .cargo/config.toml.) --target should be allowed multiple times, and/or accept multiple targets (probably comma-separated).

Describe alternatives you've considered

• Duplicate about.toml, keep the two files in sync
• Just use one about.toml that lists every platform (this is what we're currently planning on doing)