Implement reading of env-vars as config values

[ataggart]

E.g.:

{
com.example/my-var-with-default #config/env ENV_VAL ; default: 123
com.example/my-var-no-default   #config/env ENV_VAL
}

If $ENV_VAL=456 then both of the above vars should have a post-config value of "456".

If $ENV_VAL is not set, com.example/my-var-with-default will have a post-config value of 123 and com.example/my-var-no-default will be unbound.

[ataggart]

Also need to be cognizant of how this will affect config.edn generation by lein-config.

[ataggart]

Implemented in commit 1b49003

[zspencer]

@ataggart you can put "Fixes #1" in your commit body and it will auto-close once in master

[ataggart]

Ah cool, thanks!