Add .exe suffix to python executable only when necessary

[mmaday]

• Test the platform using uname and add the .exe suffix if using cygwin or mingw
• Only add and source .zshrc/.bashrc if they are available

Fixes https://github.com/qw3rtman/p/issues/11 and https://github.com/qw3rtman/p/issues/30 (which is essentially a duplicate issue)