Feature: Specify package load order

[tomory]

Some packages require other packages to be loaded first. (For example, 'preservim/vim-markdown' can utilize functions defined in 'godlygeek/tabular', but only if Vim loaded tabular before vim-markdown.) It would be good to be able to specify such a requirement when running 'pack install '.

Maybe this is already possible. If so, it would be good to mention it in the README and give an example.