Allow to run as a single file script

[dex4er]

Hi.

I love your tool because I can use it as a single file script in an environment without pip and the Internet access and I'm not forced to package it into binary archive. Dead simple deployment so it is very devops friendly.

I use only this small improvement that cli.py works even if there is no other file in the system (__init__.py in this case). Ok, a hashbang at top is quite useful too.

Then you can decide if you want to install the project standard way (pip install ...) or just upload this one file to the target host (scp cli.py host:/usr/local/bin/jinja2-cli).