feat: get and getOrNil template funcs to allow defaulting in templates (#370)

* feat: `get` and `getOrNil` template funcs to allow defaulting in templates

Ref #357

* Add docs about missing keys and default values in templates

docs/writing-helmfile.md

@@ -2,6 +2,34 @@
 
 This guide covers the Helmfile’s considered patterns for writing advanced helmfiles. It focuses on how helmfile should be structured and executed.
 
+## Missing keys and Default values
+
+helmfile tries its best to inform users for noticing potential mistakes.
+
+One example of how helmfile achieves it is that, `helmfile` fails when you tried to access missing keys in environment values.
+
+That is, the following example let `helmfile` fail when you have no `eventApi.replicas` defined in environment values.
+
+```
+{{ .Environment.Values.eventApi.replicas | default 1 }}
+```
+
+In case it isn't a mistake and you do want to allow missing keys, use the `getOrNil` template function:
+
+```
+{{ .Environment.Values | getOrNil "eventApi.replicas" }}
+```
+
+This result in printing `<no value` in your template, that may or may not result in a failure.
+
+If you want a kind of default values that is used when a missing key was referenced, use `default` like:
+
+```
+{{ .Environment.Values | getOrNil "eventApi.replicas" | default 1 }}
+```
+
+Now, you get `1` when there is no `eventApi.replicas` defined in environment values.
+
 ## Layering
 
 You may occasionally end up with many helmfiles that shares common parts like which repositories to use, and whichi release to be bundled by default.