init_printing: Fix pretty_print=False for ipython.

Allow for pretty_print=False to completely disable pretty printing.
Use formatter.for_type instead of formatter.for_type_by_name which
seems to be buggy. Fixes issue 3604 and also cleans up the code.
Don't start ipython with init_session().

By default, don't start ipython with init_session() launched from
standard python. Launching ipython can still be requested explicitly
with init_session(ipython=True).

The logic to try to load ipython if available else falls back to
standard python is moved to bin/isympy.

Fixes issue 3292.