ask for pass if missing or use env pass, add token support (#60)

README.md

@@ -113,6 +113,7 @@ Configure `esctl` with environment variables:
 - `ES_URI`: elasticsearch uri
 - `ES_USER`: username
 - `ES_PASS`: password
+- `ES_TOKEN`: API token, instead of user+password
 
 Or create a config file such as this:
 
@@ -121,11 +122,10 @@ clusters:
   foobar:
     uri: https://es.foo.bar:9200/
     user: elastic
-    pass: 123456
+    pass: ******
   other:
     uri: https://myes.foo:9200/
-    user: elastic
-    pass: asdasdasd
+    token: ******
 ```
 
 and specify it with `-c configfile`.  You may also put clusters into a