Annotate on_delete/on_update foreign key constraints

[sashabelozerov]

I find it very useful to have on_delete and on_update FK constraint values in annotations.

e.g.

fk_rails_02e851e3b7 (foreign_thing_id => foreign_things.id) ON DELETE => nullify ON UPDATE => cascade

in case of -f makrdown option:

fk_rails_02e851e3b7 (ON DELETE => nullify ON UPDATE => cascade)

[coveralls]

Coverage increased (+0.2%) to 72.939% when pulling 7cf6457 on sashabelozerov:on_delete_on_update into 91da93d on ctran:develop.

[ctran]

Thanks!