Variables and templates

Support config variables, variable templates and variables from the environment.

Example config:
```yaml
version: 1.0
config:
  host:
    env: 'WEB_TEST_HOSTNAME'
  vars:
    api_prefix: '/api/v1/json'

cases:
  - name: test_case_1
    tests:
      - name: test_1
        url:
          template: '{api_prefix}/test'
```