Auto-import Prelude in PSCI

Newcomers to PureScript expect to be able to do

    $ pulp init
    $ pulp psci
    > 1 + 1
    2

Pulp is a good place for user-convenience things like this. Perhaps `pulp init` could add "import Prelude" to a .psci file, which is executed to initialize the psci environment.

I recall a .psci file being supported by psci in the past, but it's been removed, I believe.