Add option to customize insertion points for completions

[c-bata]

Summary

This PR fixed #53

By using this feature, we can complete items which is not only separated by space. For example, we can complete files on your local machine like:

What should we define as default word separators

In #66, Chyroc suggest that "," should have the same effect with . I checked bash-shell's word separator by running Ctrl + W (Cut the word before the cursor) after input following text which includes space, comma, period, colon, semi-colon and em space.

$ abc def,hij.klm;opq　rsu:vwx
                            . <- cursor is here

then Ctrl + W removes until space. The result is below:

$ abc
    .

From this behavior, I think we should only set space as a default word separator. This is the same as ever.

Interface design (still not implemented)

There is the scene to customize word separator.

Add prompt.OptionCompletionWordSeparator(x string) for #53 and #61.

These methods are used when we want to change the insertion points for completer.

func main() {
	p := prompt.New(
		executor,
		completer,
		prompt.OptionPrefix(">>> "),
		prompt.OptionCompletionWordSeparator(" /"),
	)
	p.Run()
}

Provide text query functions by arbitrary word separator for #66

These methods are available from KeyBind functions like:

var emacsKeyBindings = []KeyBind{
	// Cut the Word before the cursor.
	{
		Key: ControlW,
		Fn: func(buf *Buffer) {
			buf.DeleteBeforeCursor(
				len([]rune(buf.Document().GetWordBeforeCursorUntilSeparatorIgnoreNextToCursor(" ,")))
			)
		},
	},
        ...
}