Add support for path variables

[andrej-urvantsev]

That's a feature request, which is once implemented can attract humongous user base and therefore contributors to the project as well.

As mentioned in original request here Kong/insomnia#519 a lot of users stay on Postman only because of that one missing feature.

There were many discussion how to implement this and how to do that right. There are a lot of different opinions.

My suggestion is to just follow current design: create a new tab "Path var" or alike and copy-paste "Query" tab and make it work.

Once implemented there could be discussions how to improve it, redesign and etc. But the main thing - get it in Insomnium.

[andrej-urvantsev]

Ah, that's probably duplicate of #14