hw4 shell-tricks

Homework 4. 
 - 5 sections created (manage, help, history, sed, convert);
- changed order by importance;
- added pictures, keeping the same pattern ;
- improved 'sed' section, leaving enough space to a future change if you want to link the regex tutorial.

That would be the last submission, but if you think I still need some improvements. I would like your feedback so I could change some features.