Add browser-style text encoding detection

[blyxxyz]

• If there's no explicit encoding it's detected using chardetng, which was made for Firefox.
• HTML is never assumed to be UTF-8, that has to be declared explicitly. This is a bit iffy, it'll definitely break some real world output. But it's consistent with how browsers handle it. See https://hsivonen.fi/utf-8-detection/.
  One argument against this is that fetch and XHR can assume UTF-8 even if loading as a normal webpage can't. What do you think?
• BOM sniffing is no longer used when the encoding is explicit.
• Streaming and non-streaming decodes now behave the same when it comes to encoding detection and BOM handling.

I had to do a bunch of research for this one, I hope it's not too hard to follow.

[ducaale]

HTML is never assumed to be UTF-8, that has to be declared explicitly. This is a bit iffy, it'll definitely break some real world output. But it's consistent with how browsers handle it. See https://hsivonen.fi/utf-8-detection/.
One argument against this is that fetch and XHR can assume UTF-8 even if loading as a normal webpage can't. What do you think?

Being consistent with how browsers handle it seems reasonable to me. If has_charset_utf8() and chardetng both fail, users will always be able to set the charset with --response-charset.

[blyxxyz]

I think I changed my mind about HTML and UTF-8. HTTPie specializes in APIs, and you'd usually talk to those using some method that can assume UTF-8.

[ducaale]

Can you update the commit description, especially the bit about HTML and UTF-8?

[ducaale]

Approved 🎉

Can you also update the Pull request's description? thanks