html version of the cheat sheet

[fyears]

Hi, I created a HTML version of the cheat sheet.

Why:

1. Not everyone has libreoffice (a huge-size software) to edit the .odt file. But many people are likely to have a text editor and a web browser. HTML is "plain text format" instead of "binary format".
2. With HTML, it's possible to precisely control every elements' positions and styles, like font family, etc.
3. It's easier to translate the cheat sheet to another language from HTML version than odt or pdf version.
4. HTML can be viewed online, too.
5. HTML can be exported to pdf (using web browser) and image easily.

For this pull request's html version, I had tried my best to respect the visual effects of original pdf version. Still some styles are not exactly the same, but the overall effects seem good enough (to me).

The web page is developed under and has best visual effects on latest Chrome or Firefox. It's not guaranteed to work on other browsers.