make step navigation extensible

The function feature-find-step-definition passes project root, step
definition file, and step definition line number it found to a lambda
expression.
Merge pull request #59 from jesg/feature/extend-feature-goto-step-def

make step navigation extensible
fix feature-goto-step-definition

The first call in feature-goto-step-definition must be "interactive."  Lambda
expressions should be called with funcall when the expression is passed to a
function.
Merge pull request #60 from jesg/fix/feature-goto-step-def

fix feature-goto-step-definition