[suggestion]add -output to command usage description

[huahuayu]

Interesting project!

When I first try, I followed the usage, I omitted the -output flag, I think the output pic will appear in the input dir, but I didn't find it

shiming@pro ➜  notpro git:(master) notpro -input /Users/shiming/Desktop/2017-12-23_18-02-13.jpg
2019/03/19 11:09:13 Width: 500, Height: 499

Then I checked the main.go and found there's -output option, I think it's better to add -output option to usage :
notpro -input head.jpeg -output notpro.jpg

[huahuayu]

newhead.png is added to project root dir, which is not prompted to the user