Make install.sh more flexible.

Now the destination directory can be specified for each source file. Now
that this new functionality is in place, this commit also moves my i3
config to a more sensible place
Make .xinitrc executable
Remove useless comment from .vimrc

At one point I thought $PAGER interfered with Vim's manpage plugin and
needed to be cleared. It turns out that is not the case, so this comment
is more meaningless than ever.
Small usability improvements in .zshenv

  * Ensure the view command opens Vim with the appropriate options
  * Don't allow duplicates in $PATH