Support expressions in string template parameters

[coolbluewater]

I've tried ${variable_name} inside run_cmd!, that works fine.
Unfortunately ${foo.bar()} produces an error.
It's easy to work around this by setting a variable to the expression and using the variable as a template parameter.
However, this reduces the effectiveness of the macro in making the code neater.

So, be great if this could be added. Thanks for the library.