Support executable event file

[mgwilliams]

Instead of an event.json file, this allows any executable to be passed to python-lambda-local as the event path, which will be executed and its stdout used as the event.

[yxd-hde]

subprocess module is not compatible with python27. Is it possible to use a compatible way for both python2 and python3?

[clbarnes]

IMO the better way to do this would be to just support stdin if the event argument isn't given. Then you can use your regular shell utilities to pipe the event into the python-lambda-local call.

Fortunately, in the time since this PR was opened, python 2.7 was deprecated (so nobody should be using it anyway). Anyway, python 2.7 does support subprocess, you just have to use call instead of run.