Add a tea.Sequentially command

Signed-off-by: Carlos Alexandro Becker <caarlos0@gmail.com>

commands_test.go

@@ -0,0 +1,56 @@
+package tea
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestSequentially(t *testing.T) {
+	var expectedErrMsg = fmt.Errorf("some err")
+	var expectedStrMsg = "some msg"
+
+	var nilReturnCmd = func() Msg {
+		return nil
+	}
+
+	tests := []struct {
+		name     string
+		cmds     []Cmd
+		expected Msg
+	}{
+		{
+			name:     "all nil",
+			cmds:     []Cmd{nilReturnCmd, nilReturnCmd},
+			expected: nil,
+		},
+		{
+			name: "one error",
+			cmds: []Cmd{
+				nilReturnCmd,
+				func() Msg {
+					return expectedErrMsg
+				},
+				nilReturnCmd,
+			},
+			expected: expectedErrMsg,
+		},
+		{
+			name: "some msg",
+			cmds: []Cmd{
+				nilReturnCmd,
+				func() Msg {
+					return expectedStrMsg
+				},
+				nilReturnCmd,
+			},
+			expected: expectedStrMsg,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if msg := Sequentially(test.cmds...)(); msg != test.expected {
+				t.Fatalf("expected a msg %v but got %v", test.expected, msg)
+			}
+		})
+	}
+}