proper use of Misaka's HTML render flags (fix malicious HTML injection)

This commit now sanitizes *all* HTML tags written by the user (also
prevents auto-link to "unsafe" web protocols and images) as intended.

Fortunately because of Sundown's typography support, it did not affect
JS injection, but custom style tags and iframes.

PS: thanks to the anonymous submitter of a comment including a style tag
for 24pt, red font ;-)
allow raw HTML markup for a few (whitelisted) tags

To be compatible with comments from Disqus (and users unfamiliar with
Markdown), Misaka no longer disables user-inputted HTML, but the
generated HTML is now post-processed and all "unsafe" tags (not
possible with Markdown) are discarded.

Whitelist: p, a, pre, blockquote, h1-h6, em, sub, sup, del, ins, math,
           dl, ol, ul, li

This commit also removes an unnecessary newline generated by
Misaka/Sundown.