Use opaque type for auth token

So we cannot accidentially output it, as it does not implement
`Display`.

src/auth.rs

@@ -7,7 +7,6 @@ impl AuthToken {
     pub fn access(&self) -> &str {
         &self.0
     }
-
 }
 
 pub fn get_token_from_command(command: &str) -> Result<AuthToken, String> {

src/provider/github.rs

@@ -1,12 +1,12 @@
 use serde::Deserialize;
 
+use super::auth;
 use super::escape;
 use super::ApiErrorResponse;
 use super::Filter;
 use super::JsonError;
 use super::Project;
 use super::Provider;
-use super::auth;
 
 const PROVIDER_NAME: &str = "github";
 const ACCEPT_HEADER_JSON: &str = "application/vnd.github.v3+json";

src/provider/gitlab.rs

@@ -1,12 +1,12 @@
 use serde::Deserialize;
 
+use super::auth;
 use super::escape;
 use super::ApiErrorResponse;
 use super::Filter;
 use super::JsonError;
 use super::Project;
 use super::Provider;
-use super::auth;
 
 const PROVIDER_NAME: &str = "gitlab";
 const ACCEPT_HEADER_JSON: &str = "application/json";