Allow having only one env conversion (#548)

Allows setting only `from_string` or `to_string` in `env_conversions`
config. Previously, both were required.

crates/nu-command/src/env/env_command.rs

@@ -12,7 +12,7 @@ impl Command for Env {
     }
 
     fn usage(&self) -> &str {
-        "Display current environment"
+        "Display current environment variables"
     }
 
     fn signature(&self) -> nu_protocol::Signature {