add output of mix tasks to input

[RobertDober]

Given a mix task bellum_gallicum that produces the following output

     Ominia Gallia divisa est in partes tres

and a template some_file.md.eex with

   <%= xtra.taks("bellum_gallicum") %>

and running the mix task mix xtra some_file.md

then the file some_file.md shall contain

     Ominia Gallia divisa est in partes tres