Allow environment variables substitition in yaml files

It would be useful if environment variables substitution was allowed for yaml files. An usage example would be refering to a different table according to the environment we are:

``` yml
entities:
MY_TABLE:
source_database: BIGQUERY
table_name: ${GOOGLE_CLOUD_PROJECT}.mydataset.mytable
...

# or

rule_bindings:
MY_BINDING:
entity_uri: bigquery://projects/${GOOGLE_CLOUD_PROJECT}/datasets/mydataset/tables/mytable
...
```