source: add 'pkg' delegator
re-alphebetize string functions
lib: add destination stack helpers

rather than setting values manually, bork has a "destination stack" that you can
push to or pop from. This is a directory *above* where you want things to go, so
you can do for example:

destination_push $HOME/code
github mattly/dotfiles          # clones into $HOME/code/dotfiles
destination_push $(destination)/.vim/bundle
github tpope/fugitive           # clones into $HOME/code/dotfiles/vim/bundle
destination_pop
github mattly/email             # clones into $HOME/code/email
destination_pop                 # resets the destination to $PWD