Add LOG_CHARSET to declare the output charset

Allow folks to declare a charset for the rendered pages, because the browser's
fallback charset may not match the encoding used in the channels.  Ideally the
incoming data would also be decoded on recipt and then re-encoded to
LOG_CHARSET before saving, but since all my channels use UTF-8, I don't have
to worry about that.

Docs for:
* HTTP's charset parameter [1].
* HTML5's meta charset [2,3].

The MDN page also points out [3]:

> This `<meta>` element must be inside the `<head>` element and within the
> 512 first bytes of the page, as some browsers only look at these
> first bytes before choosing a character set for the page.

which is why I've made it the first entry in `<head>`.

[1]: http://www.w3.org/International/O-HTTP-charset
[2]: http://www.w3.org/TR/html-markup/meta.charset.html
[3]: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta