RFE: add support for external task.

[kareled]

Hello,
rtic since 0.6 should support externally defined tasks. However while using your setup, I'm not able to find out how to get Context from minimal.rs for let say task1 to be imported into the task1.rs -- if just this is externalized.n E.g. task1.rs is create the same directory where lib.rs is located and contains just code for async task1 from minimal.rs.
Thanks!