Make it easier to extend functionality in other packages

[Richard Cotton]

Think more carefully about assert_engine interface (particularly for is_ fns with multiple args). Write a vignette on extending assertive.

[Richard Cotton]

Partial solution via https://bitbucket.org/richierocks/assertive/commits/d7cf1dbdd6aa26a59a9a275eb355f174ad89072b

Vignette still needs writing.