parse_config doesn't allow to add extra variables

[QtRoS]

Want to contribute to DeepPavlov? Please read the contributing guideline first.

What problem are we trying to solve?:

1. `parse_config` function from `deeppavlov.core.commands.utils` doesn't allow me to add extra vars or override existing. The only way to override vars is to add environment variable. It is very unhandy. I can rewrite this function so it would allow to add extra vars. 
2. Variables in config-files are substituted by hand. Why don't you use industry standard template engines like J2? 

How can we solve it?:

1. Via adding new parameter to function. 
2. Via using jinja

Are there other issues that block this solution?:

As far as I can see - none.

If you are ok with it - I will code it myself and do PR.

[anilhimam17]

Hey if you want to add extra variables to the function you try using decorators.
Through decorators you can add additional functionality and variables to the code.