emacs: use ido-vertical-mode

displays ido options vertically. this is much less cluttered and
therefore easier to see the current and next options. also enable
sensible keybindings for vertical mode, i.e. allow C-n and C-p, and
arrow keys to navigate through options.
emacs: add projectile

been aware of this for a while but avoided using it until now partly due
to its large scope. I don't have to start using all of it at once, just
start with a couple commands which definitely look good e.g.:

projectile-find-file - use ido to open any file in the project
projectile-ibuffer   - open ibuffer with only the current project's
                       buffers (maybe replacing ibuffer-vc?)
projectile-grep      - save seconds when grepping the entire project by
                       avoiding having to choose the directory first

gradually see which of its other features also look good