Default to the canonical setting names [#66]

The goal with these changes was to give users a clearer, more readable
interface, so we should present that interface up front and only
document the aliases as a backward-compatibility option.

I've renamed the envconfig tags to reflect the switch, but I left the
actual field names the same. I think they're sufficiently meaningful
inside the code, and leaving them unchanged avoids making a bunch of
churn in the rest of the code.

README.md

@@ -23,7 +23,7 @@ steps:
   - name: lint
     image: pelotech/drone-helm3
     settings:
-      helm_command: lint
+      mode: lint
       chart: ./
 ```
 
@@ -34,12 +34,12 @@ steps:
   - name: deploy
     image: pelotech/drone-helm3
     settings:
-      helm_command: upgrade
+      mode: upgrade
       chart: ./
       release: my-project
     environment:
-      API_SERVER: https://my.kubernetes.installation/clusters/a-1234
-      KUBERNETES_TOKEN:
+      KUBE_API_SERVER: https://my.kubernetes.installation/clusters/a-1234
+      KUBE_TOKEN:
         from_secret: kubernetes_token
 ```
 
@@ -50,11 +50,11 @@ steps:
   - name: uninstall
     image: pelotech/drone-helm3
     settings:
-      helm_command: uninstall
+      mode: uninstall
       release: my-project
     environment:
-      API_SERVER: https://my.kubernetes.installation/clusters/a-1234
-      KUBERNETES_TOKEN:
+      KUBE_API_SERVER: https://my.kubernetes.installation/clusters/a-1234
+      KUBE_TOKEN:
         from_secret: kubernetes_token
 ```
 
@@ -74,6 +74,15 @@ drone-helm3 is largely backwards-compatible with drone-helm. There are some know
     * `canary_image`
     * `client_only`
     * `stable_repo_url`
+* Several settings have been renamed, to clarify their purpose and provide a more consistent naming scheme. For backward-compatibility, the old names are still available as aliases. If the old and new names are both present, the updated form takes priority. Conflicting settings will make your `.drone.yml` harder to understand, so we recommend updating to the new names:
+    * `helm_command` is now `mode`
+    ° `helm_repos` is now `add_repos`
+    * `api_server` is now `kube_api_server`
+    * `service_account` is now `kube_service_account`
+    * `kubernetes_token` is now `kube_token`
+    * `kubernetes_certificate` is now `kube_certificate`
+    * `wait` is now `wait_for_upgrade`
+    * `force` is now `force_upgrade`
 
 Since helm 3 does not require Tiller, we also recommend switching to a service account with less-expansive permissions.