Add multiple cursor support

This commit creates new keybindings and actions to handle multiple
cursors.

Here are the defaults:

    "Alt-n": "SpawnMultiCursor",
    "Alt-p": "RemoveMultiCursor",
    "Alt-c": "RemoveAllMultiCursors",
    "Alt-x": "SkipMultiCursor",

cmd/micro/buffer.go

@@ -28,7 +28,8 @@ type Buffer struct {
 	// This stores all the text in the buffer as an array of lines
 	*LineArray
 
-	Cursor Cursor
+	Cursor  Cursor
+	cursors []*Cursor // for multiple cursors
 
 	// Path to the file on disk
 	Path string
@@ -169,6 +170,8 @@ func NewBuffer(reader io.Reader, size int64, path string) *Buffer {
 		file.Close()
 	}
 
+	b.cursors = []*Cursor{&b.Cursor}
+
 	return b
 }