Allow whitelist for the context parameter check (#616)

* Allow a whitelist for the context parameter check

This allows users to configure a set of types that may appear before
`context.Context`.

Notably, I think this rule is useful for allowing the `*testing.T` type
to come before `context.Context`, though there may be other uses (such
as putting a tracer before it, etc).

See #605 for a little more context on this.

Fixes #605

* Save a level of indentation in context-as-arg validation

We can unindent if we make the above check more specific

* refactoring taking into account chavacava's review

Co-authored-by: chavacava <salvadorcavadini+github@gmail.com>

test/context-as-argument_test.go

@@ -0,0 +1,19 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/mgechev/revive/lint"
+	"github.com/mgechev/revive/rule"
+)
+
+func TestContextAsArgument(t *testing.T) {
+	testRule(t, "context-as-argument", &rule.ContextAsArgumentRule{}, &lint.RuleConfig{
+		Arguments: []interface{}{
+			map[string]interface{}{
+				"allowTypesBefore": "AllowedBeforeType,AllowedBeforeStruct,*AllowedBeforePtrStruct,*testing.T",
+			},
+		},
+	},
+	)
+}