Get `slimr_template()` to work when called inside other `slimr` verbs.

[rdinnager]

It currently works inside slimr_inline if delay = TRUE I think, but doesn't work anywhere inside slimr_output(). Could be useful to use it in the do_every argument for example.

[rdinnager]

slimr_inline() usage can be dealt with by new slim_script_render() inlines argument (see #52).