Select tool in toolbar on startup

[TD-Sky]

Implementation of https://github.com/gabm/Satty/issues/21

There are some points I need to explain:

• I defined a struct Tools (in crate::command_line) copied from crate::tools::Tools for the latent demand of generating completion scripts in future.
• I utilized clap doc-comments. It will strip the end .s of help information so I striped them manually.
• I optimize the code according to rust linter clippy within a commit. The new code is equivalent to the old, you don't need to spend too much time to review them.

[TD-Sky]

To generate completion scripts, a self-described command line parser which doesn't depend on other modules in crate is necessary. It would be reused in build.rs as a module of build script. You can reference my code .

Clap also allows to generate completion scripts at runtime of CLI, but compile time generation which uses build.rs is more convenient for CI/CD. I chose the latter.

I could make a PR for CI/CD and completion scripts this weekend if you need.

[gabm]

got it.. thank you for explaining. If you feel like, i would be very happy for that contribution