command/fix: add synopsis option to fixers

This will be used in help output and such later

command/fix/fixer.go

@@ -6,6 +6,10 @@ type Fixer interface {
 	// in some way, and returns the new, transformed structure. The
 	// Fix method is allowed to mutate the input.
 	Fix(input map[string]interface{}) (map[string]interface{}, error)
+
+	// Synopsis returns a string description of what the fixer actually
+	// does.
+	Synopsis() string
 }
 
 // Fixers is the map of all available fixers, by name.

command/fix/fixer_createtime.go

@@ -49,3 +49,7 @@ func (FixerCreateTime) Fix(input map[string]interface{}) (map[string]interface{}
 	input["builders"] = tpl.Builders
 	return input, nil
 }
+
+func (FixerCreateTime) Synopsis() string {
+	return `Replaces ".CreateTime" in builder configs with "{{timestamp}}"`
+}

command/fix/fixer_iso_md5.go

@@ -41,3 +41,7 @@ func (FixerISOMD5) Fix(input map[string]interface{}) (map[string]interface{}, er
 	input["builders"] = tpl.Builders
 	return input, nil
 }
+
+func (FixerISOMD5) Synopsis() string {
+	return `Replaces "iso_md5" in builders with "iso_checksum"`
+}

command/fix/fixer_virtualbox_gaattach.go

@@ -57,3 +57,7 @@ func (FixerVirtualBoxGAAttach) Fix(input map[string]interface{}) (map[string]int
 	input["builders"] = tpl.Builders
 	return input, nil
 }
+
+func (FixerVirtualBoxGAAttach) Synopsis() string {
+	return `Updates VirtualBox builders using "guest_additions_attach" to use "guest_additions_mode"`
+}