add option to avoid logger add current time

[nbari]

The default logger writes to file using the current time as a prefix, for example:

2017-07-30T02:27:33.285542198Z 2017/07/30 02:27:33  [http://sign-io.appspot.com/] 200 946 806.40384ms 

But when the app add its own format it may be desired to just write to the log exactly that the app is sending.

probably add a timestamp bool option:

log:
    file: /var/log/app-1.log
    age: 86400 # seconds
    num: 7     # int
    size: 1    # MegaBytes
    timestamp: false

[nbari]

Fixed on 0.15.0

if timestamp set to true it will prefix with time