Filter journalctl per unit and update project to Go 1.12 using modules

[ruben-ortiz-buybay]

One of the requirements to implement is to move logs for some specific systemctl service (basically, split between system logs and application logs)

What is included in this PR:

• update the project to Go 1.12
• using Go modules to manage dependencies (keeping the original dependencies)
• golint friendly code
• added a new configuration parameter to filter multiple units. Not using the parameter will keep the original behaviour of the service
• update documentation with the new configuration

[fleaz]

Hey,
Thanks for this PR!
I just merged this into my fork because I wanted to have this feature and found an issue: When you want to mimic the use of journalctl -u foo you need to use _SYSTEMD_UNIT= instead of SYSLOG_IDENTIFIER= in line 28 of journal.go. To filter on SYSLOG_IDENTIFIER one would use journalctl -t foo and the two values are not always the same. See this commit for the changes I made.