Add option to pass integration commands through filter elements

Summary

Depending on a filter element will strip the integration commands of the element it filters, which (in my opinion) is not the behaviour one would expect. For example, I want to filter out the produced static libraries from some package, but still have things integrated nicely at runtime. This is currently very awkward to achieve.

Steps to reproduce

Create some foo.bst with some integration commands, create a filter-foo.bst filter element with foo.bst as its sole dependency. If we depend on filter-foo.bst instead of foo.bst, the integration commands will not be run.

What is the current bug behavior?

Integration commands do not get preserved by filter elements.

What is the expected correct behavior?

Integration commands are passed seamlessly through filter elements, provided an option is set to make this happen.

Edited by Tom Coldrick