Checkbox to enable optional rules

[l0b0]

I filed koalaman/shellcheck#2590 by accident, after not realising the web page doesn't enable optional rules. It would be really useful to have some way to enable such rules, and it might be useful to change the shellcheck.net link in https://github.com/koalaman/shellcheck/blob/master/.github/ISSUE_TEMPLATE.md to point to a page with all rules enabled.

[koalaman]

As in a real script you can use a directive, # shellcheck enable=all, though it's obviously not a very discoverable feature on the website.