Extract separate prepare function

[ecwyne]

Re #62 #63

import {prepare} from '@gramps/gramps';

const {schema, context, addContext} = prepare(...);
// do stuff with `schema`
app.use('/graphql', addContext) // req.gramps = {...[gramps context stuff]}

[jlengstorf]

Looks good. Thanks @kbrandwijk and @ecwyne!