Add support for the dcim/_choices endpoint

[Ichabond]

Netbox has a _choices endpoint, which is currently not supported, as it gets remapped to -choices in endpoint.py (https://github.com/digitalocean/pynetbox/blob/master/pynetbox/lib/endpoint.py#L62), and it doesn't map to the default Record structure.

Update the application so it no longer remaps endpoints beginning with _, and add a specific class Choice that only has an items() function to retrieve the results.

[jord-bh]

@zachmoody any chance this will be bundled into pynetbox soon? I've solved this manually by the requests lib in my scripts for the time being, having it available straight through pynetbox would be sweet!

And obviously, while we're at it, having it available for all objects (dcim/virtualization/ipam/...) would be extra sweet.

[ebusto]

I'm also very interested in this!