Add variables property to template frontmatter

[ezhikov]

Hello.

So, my proposed solution to #61 is to add property variables to frontmatter. Other arguments have priority over variables. One thing that I'm uncertain with, if variables should be calculated depending on other variables.

If my solution satisfies you, I'll add relative section to docs and tests if you'llguide me on how to properly test this feature.

[jondot]

@ezhikov any possibility to add tests? LMK if you need help with understanding how testing works here

[ezhikov]

I think I do need guidance with tests.

[epszaw]

@ezhikov it's very simple. Look at other test a try to do the same. If you don't have any experience with unit testing and test tools, I highly recommend to read jest docs. This framework provide the most of important testing conceptions and you can learn really a lot of cool things 👍