Implement syntax for restricted choices

[parisk]

env-spec-py should support restricting the available values of an environment variable.

Example

Input

DEBUG: [0,1]

Output

<label for="env_spec_admin_debug">DEBUG</label>
<select id="env_spec_admin_debug" name="admin_debug">
  <option value="0">0</option>
  <option value="1">1</option>
</select>

Spec

https://github.com/sourcelair/env-spec#restricted-choices

[parisk]

Closed in #13.