feat: tea.WithContext ProgramOption to supply a context

WithContext lets you specify a context in which to run the Program.
This is useful if you want to cancel the execution from outside.
When a Program gets cancelled it will exit with an error
ErrProgramKilled.

options.go

@@ -1,6 +1,7 @@
 package tea
 
 import (
+	"context"
 	"io"
 
 	"github.com/muesli/termenv"
@@ -14,6 +15,15 @@ import (
 //	p := NewProgram(model, WithInput(someInput), WithOutput(someOutput))
 type ProgramOption func(*Program)
 
+// WithContext lets you specify a context in which to run the Program. This is
+// useful if you want to cancel the execution from outside. When a Program gets
+// cancelled it will exit with an error ErrProgramKilled.
+func WithContext(ctx context.Context) ProgramOption {
+	return func(p *Program) {
+		p.ctx = ctx
+	}
+}
+
 // WithOutput sets the output which, by default, is stdout. In most cases you
 // won't need to use this.
 func WithOutput(output io.Writer) ProgramOption {

tea.go

@@ -141,8 +141,14 @@ func NewProgram(model Model, opts ...ProgramOption) *Program {
 		msgs:         make(chan Msg),
 	}
 
+	// A context can be provided with a ProgramOption, but if none was provided
+	// we'll use the default background context.
+	if p.ctx == nil {
+		p.ctx = context.Background()
+	}
+
 	// Initialize context and teardown channel.
-	p.ctx, p.cancel = context.WithCancel(context.Background())
+	p.ctx, p.cancel = context.WithCancel(p.ctx)
 
 	// Apply all options to the program.
 	for _, opt := range opts {

tea_test.go

@@ -2,6 +2,7 @@ package tea
 
 import (
 	"bytes"
+	"context"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -97,6 +98,28 @@ func TestTeaKill(t *testing.T) {
 	}
 }
 
+func TestTeaContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	var buf bytes.Buffer
+	var in bytes.Buffer
+
+	m := &testModel{}
+	p := NewProgram(m, WithContext(ctx), WithInput(&in), WithOutput(&buf))
+	go func() {
+		for {
+			time.Sleep(time.Millisecond)
+			if m.executed.Load() != nil {
+				cancel()
+				return
+			}
+		}
+	}()
+
+	if _, err := p.Run(); err != ErrProgramKilled {
+		t.Fatalf("Expected %v, got %v", ErrProgramKilled, err)
+	}
+}
+
 func TestTeaBatchMsg(t *testing.T) {
 	var buf bytes.Buffer
 	var in bytes.Buffer