Allow variables to be embedded in the desc section

[k2tzumi]

vars:
  username: "foo"
steps:
- desc: "Retrieve a list of {{vars.username}} articles"
  req:
    /api/articles?username={{ vars.username }}&count=96&order=latest:
      get:
        body:
          application/json: null

[k1LoW]

LGTM!

This feature may be somewhat of a bottleneck when trying to statically generate documentation without running a runbook.
However, we believe that the decision to use this feature or to generate documentation should be left to the user.

[k2tzumi]

I see. So this is a case of static analysis to get only the description.