Add option to use env vars for variables

[colmsnowplow]

It would be a nice convenience feature to be able to just set env vars for the playbook vars I want to use, then use some kind of flag in the sql-runner command (eg. -env-vars).

This way each target's config can be in a bash profile, and there's less hassle in running commands.

[colmsnowplow]

With further experimentation it seems that vars potentially can't be supplied for the 'targets' playbook section, only the 'variables' one (unless I'm doing something wrong) - so perhaps this requirement is to implement the ability to fill any template in the playbook, and also do so via env vars.

[alexanderdean]

See also https://github.com/snowplow/sql-runner/issues/149