[feature] Add support for generating shell completions

[Brezak]

There are 2 ways this could be done:

• A build script like dust does it
• A flag similar to how mdBook does it

I've tried to implement both solutions and the build script solution would require changing how imports are done since just including the context module essentially means having to include almost the entire project module hierarchy.

[solidiquis]

Thanks @Brezak for tackling this! I'll do my best to take a look at this and give you my thoughts sometime tonight or tomorrow :]

[Brezak]

@solidiquis should have mentioned this but I've also made an attempt at implementing the flag approach if you wanna take a look at how it could work.

[solidiquis]

@Brezak I do like the flag approach as it feels far less awkward in my opinion. Not a huge fan of the duplicate code in the build.rs as it can be painful for people knowing that they need to sync up changes between that and main.rs, utils.rs etc.. Let me know your thoughts. Feel free to whip up a PR!