Custom variables for custom templates

[vdjagilev]

Add a possibility to pass custom variables for custom templates.

Example:

--x-opt "foo=${bar}"

Then this value can be used in a custom template like this:

Some custom variables:

<ul>
  <li><b>Foo value:</b> {{.custom.foo}}</li>
</ul>

This could be used in automated environments (pipelines?) to pass some values to the custom templates.

Can be implemented only after #23