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

context/context_test.go

@@ -280,3 +280,25 @@ func TestExplicitSubresourcePathLoading(t *testing.T) {
 		t.Fail()
 	}
 }
+
+func TestSetVariablesFromArguments(t *testing.T) {
+	vars := []string{"version=some-service-version"}
+	ctx, _ := LoadContextFromFile("testdata/default-loading.yaml")
+
+	if err := ctx.SetVariablesFromArguments(&vars); err != nil {
+		t.Error(err)
+	}
+
+	if version := ctx.Global["version"]; version != "some-service-version" {
+		t.Errorf(`Expected variable "version" to have value "some-service-version" but was "%s"`, version)
+	}
+}
+
+func TestSetInvalidVariablesFromArguments(t *testing.T) {
+	vars := []string{"version: some-service-version"}
+	ctx, _ := LoadContextFromFile("testdata/default-loading.yaml")
+
+	if err := ctx.SetVariablesFromArguments(&vars); err == nil {
+		t.Error("Expected invalid variable to return an error")
+	}
+}