* Allow using custom time format
I need to use custom time format in `conf/app.ini' like
FORMAT = 2006-01-02 15:04:05
so that Gitea will display '2017-01-30 08:41:49'
check this answer for more constants to format date <http://stackoverflow.com/a/20234207/2570425>
PS: First GO commit
* Refactor and validate TimeFormat (must have 2006, 01, 02, 15, 04 and 05)