Read generation options from toml file

[Robzz]

I'm getting tired with having to specify all the packages I want to document in a workspace. I'd like cargo-docset to read parameters from a toml file (if present/specified), or from the command line otherwise, maybe with the ability to override or add to the parameters from the command line.

I think all that needs to be done is to create the appropriate serializable types (one per command I imagine, and there's only the generate command for now) that we can then deserialize with e.g. serde-toml.

[Robzz]

It seems that Cargo.toml has metadata sections that could be used for this. Something that reads from the workspace.metadata table should probably be fine, for a start. The information is probably not hard to get to with the cargo_metadata crate that we already depend on.