Allow editable default for Input prompt

[mislav]

When doing

survey.Input{
	Message: "What is your name?",
	Default: "Jordan",
}

the default value "Jordan" is now editable and can be navigated around with the cursor if survey.WithShowCursor(true) was used.

Promoting visibility for this idea: #196 (comment)

@AlecAivazis Since this is backwards incompatible, what do you think about adding a separate field for editable default value? E.g. DefaultEditable: "Jordan"

[AlecAivazis]

Hm, good question. I think having a new field is "alright". it's definitely not great but we're kind of stuck with it. If there are enough little breaks maybe its time to get them together?