[feature] Defining and consuming custom resources

This is one possible solution for #406. The idea is to implement custom resources similarly to mesos:

https://mesos.apache.org/documentation/attributes-resources/
### When starting a client, specify resources it exposes

```
client {
enabled = true

resource "gpu" {
type = "range"
begin = 1
end = 4
}

resource "ip" {
type = "ip-range"
begin = "192.168.1.0"
end = "192.168.1.255"
}

resource "github_token" {
type = "set"
items = ["1234", "abcd"]
}

resource "network" {
type = "enum"
items {
private = "eth2"
overlay = "overlay1"
}
}
}
```
### Allow to specify used resources in task specification

```
task {
resources {
gpu = 2
ip = 1
github_token = 1
network = "private"
}
}
```
### Expose used resource to tasks (and custom drivers) via env variables

```
NOMAD_RESOURCE_gpu = '1,2'
NOMAD_RESOURCE_ip = '192.168.1.5'
NOMAD_RESOURCE_github_token = '1234'
NOMAD_RESOURCE_network = 'eth2'
```

Start by reviewing the proposed client resource declarations, task resource requirements, and environment-variable examples in the issue, along with the linked Mesos resource documentation. Done would require agreeing on the resource types, configuration and task syntax, allocation behavior, and task-facing environment variables.

Written by the indexing model from the issue text.