List source and database drivers in help text

cli/main.go

@@ -11,6 +11,8 @@ import (
 	"time"
 
 	"github.com/golang-migrate/migrate"
+	"github.com/golang-migrate/migrate/database"
+	"github.com/golang-migrate/migrate/source"
 )
 
 // set main log
@@ -52,7 +54,9 @@ Commands:
   drop         Drop everyting inside database
   force V      Set version V but don't run migration (ignores dirty state)
   version      Print current migration version
-`)
+
+Source drivers: `+strings.Join(source.List(), ", ")+`
+Database drivers: `+strings.Join(database.List(), ", ")+"\n")
 	}
 
 	flag.Parse()

database/driver.go

@@ -110,3 +110,14 @@ func Register(name string, driver Driver) {
 	}
 	drivers[name] = driver
 }
+
+// List lists the registered drivers
+func List() []string {
+	driversMu.RLock()
+	defer driversMu.RUnlock()
+	names := make([]string, 0, len(drivers))
+	for n := range drivers {
+		names = append(names, n)
+	}
+	return names
+}

source/driver.go

@@ -105,3 +105,14 @@ func Register(name string, driver Driver) {
 	}
 	drivers[name] = driver
 }
+
+// List lists the registered drivers
+func List() []string {
+	driversMu.RLock()
+	defer driversMu.RUnlock()
+	names := make([]string, 0, len(drivers))
+	for n := range drivers {
+		names = append(names, n)
+	}
+	return names
+}