Detect attachEvent() support correctly, fixes #771

[hallvors]

No description provided.

[khanbot]

CLA signature looks good 👍

[kohler]

Since main.js is only used in testing, I see no problem with this. It would be more self-consistent with the code above to write "attachEvent" in input, but it's fine either way.

[kohler]

I'll wait a bit to merge just in case "attachEvent" in input arrives as a PR revision.

[edemaine]

As discussed in #771, I'm not convinced this will be useful in isolation -- there's a call to attachEvent a few lines below that should be similarly protected, if this one is. But with a revision (and ideally testing to confirm this works where desired) it seems like a good idea.

[kohler]

@edemaine This is a change to a file that is only used in testing KaTeX. I am happy to trust the submitter that the change was useful in his environment.