renderClass API

[zcysxy]

Related to #31. Seems like pretty straightforward.

To test it, I duplicated the katex-samples.html, changed all the id="..." to class="pseudocode", and replaced the script with

pseudocode.renderClass(document.getElementsByClassName("pseudocode"),
                                 {
                                     lineNumber: false,
                                     noEnd: true
                                 });

If there is no problem we can make it to the docs.

[SaswatPadhi]

Thanks! The logic looks good to me, but I think the API could be improved.

Also, please add an example usage snippet to README.md in Step 4.

[zcysxy]

Thanks for the review, totally make sense! All changes are applied.

[SaswatPadhi]

Awesome, thank you so much! LGTM.