add docs about subref arguments

Subrefs also improve the profile of custom arguments, so move those up.

The dynamic API is slowly becoming obsolete. Exactly as planned. Muah ha
ha ha ha ha ha ha haaaaa.
add package arg to subref callbacks

This gets rid of the need to use caller() to figure out who is asking
for these exports. If we used caller() and Import::Base's API changed,
we could break any subref that relied on the caller level remaining
unchanged.

Fixes #19
add example of custom arguments