Add env variable for logging to Syslog

[Stebalien]

This was added in #46 and removed in #47 because windows doesn't support it. We should (maybe?) add it back under an "is unix" build flag.

[Stebalien]

We can also just not have this feature, up to you @frrist. I just figured we should have an explicit issue about this.

[eingenito]

Sorry - closed inadvertently.

[gammazero]

Consider writing to a fifo (named pipe) that syslog is configured to read from. This can be accomplished by setting the environ variable GOLOG_FILE to the path of the fifo.

If the above is reasonable, then I recommend closing this issue.