Fix filter.end()

Prior to this commit, filtered files would not be piped to the next
stream within non-filtered files. So filter.end() would not bring back
filtered files.

This commit buffers ignored files during the filter() phase and pushes
them all within a ".gulp-filter" file with null contents.
This implementation should work with most Gulp plugins, since Gulp
guidelines state that gulp plugins should just push files that verify
file.isNull().

Fixes #2