Add two API methods to set RuneBuffer state before prompting the user (#105)

* Expose RuneBuffer.Set through Operation's API

This allows a user to prefill a line before the user is given a prompt,
so that you can provide a default, user-editable input line.

* Add a Instance.ReadlineWithDefault API method

This will pre-fill the Operation.RuneBuffer with text before prompting
the user, and provide her with a method to edit the provided default.

operation.go

@@ -32,6 +32,10 @@ type Operation struct {
 	*opVim
 }
 
+func (o *Operation) SetBuffer(what string) {
+	o.buf.Set([]rune(what))
+}
+
 type wrapWriter struct {
 	r      *Operation
 	t      *Terminal

readline.go

@@ -242,6 +242,11 @@ func (i *Instance) Readline() (string, error) {
 	return i.Operation.String()
 }
 
+func (i *Instance) ReadlineWithDefault(what string) (string, error) {
+	i.Operation.SetBuffer(what)
+	return i.Operation.String()
+}
+
 func (i *Instance) SaveHistory(content string) error {
 	return i.Operation.SaveHistory(content)
 }