feat: add a template cache string parameter

[mpalpha]

I'd like to add a template cache via string parameter
So that I can get something like this:

  src: url("./webfont.eot?1565728069335");

Proposed by #191

[jimmyandrade]

@mpalpha Thanks for contributing! Is it something that you could help us build on?

@evilebottnawi What do you think of this feature request?

[alexander-akait]

I think it is good idea for simple cases, need a option for this

[jimmyandrade]

I think it is good idea for simple cases, need a option for this

@evilebottnawi I've merged PR #191 from @mpalpha, that includes template cache string support. When do you believe that we could release a version to allow people to test?