Small updates to README.md
Faster formatting of large tables

Ref #44
Merge remote-tracking branch 'upstream/master' into dev
Merge remote-tracking branch 'upstream/dev' into dev
further large table optimizations

1. s:get_aligned_rows(): getting 2 last rows is enough for having been
   formatted tables
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