Implement a pyimport string macro

[JamesWrigley]

This helps usability by automatically converting Python import statements into their Julia equivalents.

Example from the docs:

pyimport"""
import numpy as np
from scipy import linalg, optimize
from os.path import join as pathjoin
"""

# Converted to:
const np = pyimport("numpy")
const linalg = pyimport("scipy" => "lingalg")
const optimize = pyimport("scipy" => "optimize")
const pathjoin = pyimport("os.path" => "join")