Q: is it possible to define some global variables?

I use swagger ui before.
I found `oax` today, it's awsome!!

I always want to define some global variable,
  so I can use them at anyway in openapi.yml.

like the variable used in bash or javascript string:

```javascript
var1='abc123'
str1=`hello ${var1}`
```

<br>

original openapi spec seem not support this,
extensions or some hack can do it easily?
