Support attachments to internal note file.
- Add/Delete/Clear/Sort;
- Support custom attachment folder for each notebook (read-only);
- Support renaming attachment;
1. Support `/` and `?` to search. `N` and `Shift+N` to find next/previous
occurence.
2. `Ctrl+N` and `Ctrl+Shift+N` to navigate through the command history.
3. `:nohlsearch` or `<leader><space>` to clear search highlight.
4. `#` and `*` to search current word under cursor.
Different behaviors from Vim: after deleting the line with a mark set,
VNote could not detect if this mark is set or not. VNote just simply
jumps to the same line.
