New Page: Snippets Cheat Sheet

Focus to provide a quick reference on how to do general stuff in
HaxeFlixel, used in every project and all the time while coding one.
Not made for beginners, but can also be useful to them to see how
things are done, but the purpose isn't to give much explanation here
(that's what specific pages).

Needs a revision.
Update 18-snippets-cheat-sheet.html.md

Corrected all changes mentioned by @Gama11, plus some other improvements.
Now it looks beautiful :D
Update 18-snippets-cheat-sheet.html.md

All mentioned changes are done.
- `FlxText.text` in constructor: I think it's better to show how to access this field as it's of common use, right?

Let me know any other detail and sorry for so many flaws initially :S
Renamed file

* Renamed file to exclude 'snippets' title
* Many minor corrections
Update 18-cheat-sheet.html.md

- Added instruction to open debugger
- Removed useless `add()` from snippet
Moved to "Resources", updated info

* Moved to "Resources" section, set it as 2nd on the list (updated the
others respectively)
* FlxTween: updated link to DEMO.
* Added FlxTypedGroup info
Merge branch 'master' of https://github.com/AndreiRegiani/flixel-docs