refactor: let updaters define the files they want to run on (#233)

This change reverses the responsibility for which files the updaters are
run on. Now each updater can specify the list of files and wether the
files should be created when they do not exist yet. This simplifies the
handling of each update in releaserpleaser.go, as we can just iterate
over all updaters and call it for each file of that updater.

Also update the flags to allow users to easily define which updaters
should run.

internal/updater/updater.go

@@ -5,7 +5,11 @@ type ReleaseInfo struct {
 	ChangelogEntry string
 }
 
-type Updater func(content string, filename string) (string, error)
+type Updater interface {
+	Files() []string
+	CreateNewFiles() bool
+	Update(info ReleaseInfo) func(content string) (string, error)
+}
 
 type NewUpdater func(ReleaseInfo) Updater