Remove warnings in generated code

[projectgus]

Thanks for creating this project, it's exactly what I was looking for!

This PR is a small collection of individual commits to remove warnings in the generated Rust code. It shouldn't change any actual functionality:

• Disable unused imports warnings (fixes Compile warnings unused and redundant imports #53)
• Avoid warnings in code generated for messages that contain no signals (not common, but possible especially when reverse engineering.)
• Add a config option for #!allow(dead_code) in the generated source file, useful if you're generating from a large DBC file but don't intend to use all of it in your project.
• Disable the clippy too_many_arguments warning. Constructors for messages with a lot of signals are impractical, but hard to see an easy way to introduce an alternative constructor.

[killercup]

Thanks!