dpkg arbitary meta files and triggers

As suggested in #595 and #599, this adds support for dpkg triggers with `--deb-interest` and `--deb-activate`, and arbitary meta files for Debian packages with `--deb-meta-file`. If you provide a `triggers` file through `--deb-meta-file` _and_ interest/activate triggers, the latter will be appended to your provided `triggers` file after being copied into the package structure.

I do get a failing test on master (in the `debian6` Vagrant image), but all others pass:

```Failures:

  1) FPM::Package::Deb#output with lintian when run against lintian should return no errors
     Failure/Error: expect($?).to eq(0), lintian_output
       E: name: control-file-has-bad-owner conffiles 0/0 != root/root
       E: name: control-file-has-bad-owner md5sums 0/0 != root/root
       W: name: unknown-section default
       W: name: script-in-etc-init.d-not-registered-via-update-rc.d /etc/init.d/test
       W: name: init.d-script-missing-lsb-section /etc/init.d/test
     # ./spec/fpm/package/deb_spec.rb:345```

This patch has also been tested in a more "real-world" situation through our package building and deployment process.