Add company-box-buffer-hook to run on newly created buffers

[bjcohen]

Create hook to run customizations on newly created company-box buffers (both the choices buffer and the doc buffer).

Rationale - I use https://github.com/ema2159/centaur-tabs, and don't want tabs to show up in my company-box completion popup. The way to do this in Centaur is to enable centaur-tabs-local-mode in the buffer. The cleanest way to facilitate this would be either:

a) Create a new mode to run the popup buffers in, add centaur-tabs-local-mode to the mode hooks. b) Add a hook to run upon buffer creation.

I chose b), as it adds less cruft.