feat(context): allow explicit variables to be defined as argument

These changes allows variables to be defined when executing
`kontemplate` via one or more `--variable` arguments.

With this in place one can either define new variables or override
existing variables loaded from a file:

```
$ kontemplate apply --variable version=v1.0 example/fancy-app.yaml
```

This avoids the need to write variables into a temporary file that is
only needed to provide "external variables" into resource sets.

Closes https://github.com/tazjin/kontemplate/issues/122

main.go

@@ -37,6 +37,7 @@ var (
 	// Global flags
 	includes = app.Flag("include", "Resource sets to include explicitly").Short('i').Strings()
 	excludes = app.Flag("exclude", "Resource sets to exclude explicitly").Short('e').Strings()
+	variables = app.Flag("var", "Provide variables to templates explicitly").Strings()
 
 	// Commands
 	template          = app.Command("template", "Template resource sets and print them")
@@ -188,6 +189,11 @@ func loadContextAndResources(file *string) (*context.Context, *[]templater.Rende
 		app.Fatalf("Error loading context: %v\n", err)
 	}
 
+	err = ctx.SetVariablesFromArguments(variables)
+	if err != nil {
+		app.Fatalf("Error setting explicit variables in context: %v\n", err)
+	}
+
 	resources, err := templater.LoadAndApplyTemplates(includes, excludes, ctx)
 	if err != nil {
 		app.Fatalf("Error templating resource sets: %v\n", err)