Add APIs for alternate CLIs to replicate interactive environment

[osandov]

For IPython (#318) and ptpython (#309), it'd be nice if drgn provided APIs to set up the environment in the same way that the standard CLI's interactive mode does. We can factor stuff out of drgn.cli.run_interactive() and add it to drgn.cli. The main thing that comes to mind is the default imports, but there may be others.