add postfix plugin

[phemmer]

This is a plugin for providing metrics on postfix queues.
For each of the active, hold, incoming, maildrop, and deferred queues (http://www.postfix.org/QSHAPE_README.html#queues), it will report the queue length (number of items), size (bytes used by items), and age (age of oldest item in seconds).

Required for all PRs:

• CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
• Sign CLA (if not already signed)
• README.md updated (if adding a new plugin)

[danielnelson]

This just needs the README updated so it matches the style of the EXAMPLE_README.

[dirkaholic]

Any progress here ?

[abednarik]

I would love to see this, since I use telegraf everywhere

[abednarik]

Thanks @danielnelson.

If I can get some free time, I will do the missing README file here.

Cheers.

[danielnelson]

Looks like @phemmer already did this, so I think we are good.

[abednarik]

Oh yeah, you are right, thanks again.