Add bin/python that mimics Python CLI

[tkf]

This PR implements the tricky part (= libjulia initialization) of PyJulia "inside-out." By that I just mean to provide a simple Julia script (= bin/python) that drops into Python interpreter via PyCall. Inside this Python interpreter, setting up PyJulia is "free" and don't care if the Python executable is statically linked or not because it does not use the Python executable.

Now I can do

$ bin/python -c 'from julia import Base; Base.banner()'
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.0.0 (2018-08-08)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

even with Python from conda (whose sys.executable is statically linked to libpython).

[stevengj]

Can't this be done in Julia?

[stevengj]

I'm unclear on how you plan to use this in pyjulia — can you clarify?

If this is mainly intended for use in pyjulia, why does it need to be in PyCall.jl?

[tkf]

If this is mainly intended for use in pyjulia, why does it need to be in PyCall.jl?

Ah, yes, that's a good point. Historically, I started writing this patch in the form that provides PyJulia-like Python package that does not require PyJulia so it made sense to have it in PyCall. But then I totally agree that it should be in PyJulia.