enhanced default cluster selection, add 'cluster switch', overhaul json header setting (#47)

README.md

@@ -80,6 +80,7 @@ ccr                   - manage cross cluster replication
   info                - show ccr remote info
 cluster               - manage cluster[s]
   status              - show cluster status
+  switch              - set current elasticsearch cluster
   list                - list configured clusters
   settings            - cluster settings management
     list              - show cluster settings
@@ -158,7 +159,7 @@ Or create a config file such as this:
 
 ```yaml
 clusters:
-  default:
+  foobar:
     uri: https://es.foo.bar:9200/
     user: elastic
     pass: 123456
@@ -172,9 +173,37 @@ and specify it with `-c configfile`.  You may also put clusters into a
 default config file in `~/.config/esctl/config.yaml`. In this case you
 can omit `-c ...`.
 
-If you want to  work on a specific cluster, specify  its name with the
+If you want to  work on a specific cluster, you need to make it the
+current default one. You can either manually configure it in the
+config:
+
+```yaml
+clusters:
+  foobar:
+    uri: https://es.foo.bar:9200/
+    user: elastic
+    pass: 123456
+    default: true
+```
+
+or - if the cluster already exists in the config - issue this command:
+
+```console
+esctl cluster switch foobar
+```
+
+You may also temporary set a cluster as the current default with the
 global `-C` option.
 
+The following rules apply for default cluster selection:
+
+1. If there's just one cluster configured (either via environment
+   variables or config), this one will be selected.
+2. If there is one cluster configured with the `default` flag `true`,
+   use this one.
+   
+Use `esctl cluster ls` to check status and see which one would be used.
+
 ## Installation
 
 The tool does not have any dependencies.  Just download the binary for