* Update logrus to 1.2 and add as a direct dependency
* Create an mlog/human package for pretty-printing logs
This package can read JSON logs from mattermost.log, and output the data to
either logrus or a custom formatter, to make the logs more human readable.
* Create a command for outputting human-readable logs
This command will read JSON data from mattermost.log or stdin, and
output in a human readable format. An optional argument can be used
to activate logrus output (which includes color support).
* Reorganize code in mlog/human and improve logrus timestamp formatting
