Ported customize/filters/tests functionality from j2cli

[mlasevich]

Like many others we have been affected by the j2cli's lack of maintenance. Looking around this seems to be the best candidate for a drop-in replacement - but to be a complete drop-in replacement it needs to have a compatible j2cli customization mechanism. This PR ports --customize, --filters, and --tests flags from j2cli along with fixed version of the code.

I know the preferred method would normally be via a plugin, however, since plugins currently cannot handle the cli args, i am opening a PR as it is the only way to add this. I did my best to minimize code changes to cli.py and move most of the ported code is in its own source code files.

I did not do any of the documentation nor cleanup, as before I spent too much time, I wanted to first see if this is acceptable in general.

Please let me know what you need to consider merging this.

Thanks!

[kpfleming]

This is a very reasonable approach, given that I'm not likely to ever have time to implement it in the way that I had planned to :-) Feel free to continue getting this into mergeable shape, and I'll keep an eye on it so it can be merged and released.

[mlasevich]

This is a very reasonable approach, given that I'm not likely to ever have time to implement it in the way that I had planned to :-) Feel free to continue getting this into mergeable shape, and I'll keep an eye on it so it can be merged and released.

Ok, I am looking it closer and due to linter requirements, I may have to do some refactoring... I was doing my best to minimize changes to both Jinjanator and the code ported from j2cli, but my hand is forced here :-/

[mlasevich]

Best I can tell, lint/coverage/docs should all be fixed.

[kpfleming]

The code all looks great; I have a number of changes I'd like to make to the various bits of documentation, and it will be most efficient for me to just make those and push them to your branch. If that's acceptable to you, I'll get that done within the next day or so.