text: Title: `parse_config` doesn't allow to add extra variables 
  Body: Want to contribute to DeepPavlov? Please read the [contributing guideline](http://docs.deeppavlov.ai/en/master/devguides/contribution_guide.html) 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.