[WIP] Preview completion documentation and locations

[mohkale]

Lets you see the help buffers for completion candidates. Supports variants for both company-show-location and company-show-doc-buffer. There's also a separate customization variable to show the previews in a temporary popup window (defaults to true) to avoid overloading the same window where you're completing the candidate.

WARN: The location preview isn't maintained by us. It's on a backend by backend basis which means the backend could open a bunch of files as we filter the completion and then clean it up. Not a big fan of this :/. That's why the default is to show the doc-buffer instead of the completion. I'll have to look into a nicer way to support the other approach down the line.

NOTE: This PR is just to make the intention clear. I intend to keep it open for a while and experiment with the new feature before merging.