Save time entry into settings file

Also ask for entry when saving a file, with
filled text for that file.
User can ignore it or use `Esc` to skip it.

The time entries are saved as JSON with format:

```JSON
{
    "2014-01-03T17:50:19": "Finish saving entries for TimeEntry plugin ",
    "2014-01-03T17:59:51": "Asking for time entry when save a file"
}
```