Add :ggload command, which loads all `Prelude`s

The previous commit did not work, because GHCi executed the contents of
`.ghci` *before* loading the project.
This commit makes a new command, `:ggload`, available from GHCi when in
the Gargantext project: typing `:ggload` will make available all symbols
from modules named `Prelude` in the Gargantext project.
The name `:ggload` as well as the set of modules to import are still
open to discussion.