Create a named `jump` function.

Add a function `step.jump` to make explicit the action performed when
passing in a function in the cadence. Using an existing function is too
clever, I believe.

See: https://github.com/bigeasy/cadence/blob/master/diary.md#jumping-the-parent

Closes #102.

File                             Raw         Min         Zip     Min/Zip
index.js                 10926/10.67  3535/03.45  3128/03.05  1332/01.30