Add custom TagHandler for <code> tags to add monospace typeface

[DieBauer]

The textview supports formatting of a few built-in html tags,
tt (teletype) is deprecated in html5, but for android this is still implemented

Advantages of this tt formatting is that it respects newlines and whitespace using a monospace font, which makes code formatted comments on Hackernews better readable.

Hackernews, makes use of the html5 successor <pre><code> to format code blocks with a monospace font. In order to add support to the textview we need to transform the input, or adjust the presentation.

One solution takes a regex and replaces the tags for every comment.
Another solution is to implement a TagHandler that is being called when the Html.fromHtml encounters a tag that it doesn't recognize (pre and/or code), however this then involves more or less duplicating the Android standard library that formats the tt tag currently.

The latter has the advantage that it only kicks in when the comment / text contains such a tag, thus being more efficient, with the same result. And it is more futureproof.

Addresses #883 and #1037

[jblebrun]

Thanks for doing this, I was just about to investigate doing something similar, and discovered that you've already got this PR up. I would love to see this get merged in!