Feature/getenv

This adds a "getenv" function to the template. This allows a template to reference values that are passed at runtime (as in during the launch of a container). 

Note, due to https://github.com/hashicorp/consul-template/issues/64, these variables cannot be used to query consul at this time. 

Example: 

```
cluster_id: {{ getenv "CLUSTER_ID | toLower}}