install packages supplied to -Su. Closes #60.

[euclio]

I moved the code for installation after the code for updating to allow supplying packages to a -Su command. That is, it is now possible to install packages by issuing a command such as packer -Syu <packages>.This is consistent with pacman behavior.

[AladW]

if [[ foo || bar ]] is pretty ugly though. Perhaps you could use variables instead, and check for their values.