Feature Request Systemd_Journald plugin

## Feature Request
An input plugin for the systemd journal that can capture the journald logs from one or all systemd services and sockets.

### Proposal:
either use a socket or the [core os systemd library](https://github.com/coreos/go-systemd) to gather detailed log information from systemd.

### Current behavior:
In order to get some data from the systemd journal syslog or journald needs to be configured to receive logs from journald and the syslog input plugin is used gather data.

### Desired behavior
A native journald plugin that captures the detailed info directly from journald.

### Use case:
it would be nice to have one place to grab all the logs for all services instead of having to setup tail for each file or service. It would also be a great way to get logs from early in the boot process that cannot be captured from syslog.

Start by reviewing the existing syslog input plugin and the proposed coreos/go-systemd approach described in the issue. Define how the plugin should select one or all systemd services and sockets, then verify that it captures detailed journald records, including early-boot logs, without requiring syslog forwarding.

Written by the indexing model from the issue text.