Declare dependency to `outorg`

`outshine`'s dependency to `outorg` is correctly documented in the README file. However, end-users installing it via the emacs's package manager will typically not see it, leading to the situation described in issue #35.

Declaring the dependency in the package header will allow MELPA to add the dependency in the package it builds, which in turn will make `package.el` correctly install `outorg` before `outshine`, without the end users having to do anything special.