further large table optimizations

_(Please disregard changes in README.md: i do not know how did they get here and how to get rid of them.)_

1. ``s:get_aligned_rows()``: getting 2 last rows is enough for having been formatted tables: this makes adding new rows in large tables really fast
2. ``vimwiki#tbl#get_cells()``: using faster ``strpart()`` instead concatenating every new character into variables ``cell`` and ``quote``
3. checking by ``getline()`` whether the line was changed before ``setline()`` does matter on slower computers