Add support for clangd customization

[nilsdeppe]

Thanks so much for creating and maintaining emacs-ycmd! I really appreciate it!

YCMD now has experimental support for using clangd as the completer. This PR adds support for customizing the JSON file sent to YCMD with the clangd-specific options.

I'm not sure if or how this should be tested so I would really appreciate feedback on that.

Thanks!

[abingham]

I'm no longer using or actively maintaining this package. The patch you've provided looks reasonable on quick inspection, and I'd be happy to merge it in if you think it's correct. Let me know.

[nilsdeppe]

@abingham Okay, I'm going to check that the clang_args are being passed correctly, but other than that I'm confident it works. I just saw the issue about someone taking over this extension. I'll give it some thought but can probably help do some maintenance and compatibility work, though I'm far from a lisp expert :)

[abingham]

I'm going to check that the clang_args are being passed correctly,

Just let me know when you think it's ready to merge, and I'll take care of it.

though I'm far from a lisp expert :)

Believe me, I knew very little about emacs lisp when I started writing extensions. The best way to learn was to dive in. It's a lot of fun (and a little crazy)!

[nilsdeppe]

@abingham sorry for the delay on this. I just found time to double check things. I made some adjustments to the implementation so it matches what is currently done in the repo. I tested and it works.

Cheers!