Add support for 280 characters

[cfuhrman]

As Twitter has recently introduced support for 280 characters[1], bump up
maximum character limit in twittering mode. While here, make the maximum
character limit a defvar for ease of maintenance.

Closes #142

Footnotes:
[1] https://blog.twitter.com/official/en_us/topics/product/2017/Giving-you-more-characters-to-express-yourself.html

[humitos]

Nice! Looks good to me. I would like it to be merged :)

[cyborgmarina]

Thanks for that! Looks good to me as well.

[ahungry]

👍

[iRi-E]

This is incomplete solution.
Twitter now uses "weighted length" instead of max number of characters:
https://developer.twitter.com/en/docs/developer-utilities/twitter-text

A CJK character such as "あ" consumes 2 counts, so CJK tweets are still limited to 140 chars.
For example, "abcdefgh" and "あいうえ" and "abcdうえ" have same length 8.

[ahungry]

So, would 280 bytes be more correct? (I assume that symbol is a 2 byte unicode?)

[iRi-E]

No, that's not. The "weighted length" is besed on the amount of information in one character, not related to the size of unicode's byte sequence. FYI, most of Japanese characters are 3 byte sequences.

[cvmat]

Thanks @iRi-E for the URL of the document.
I have committed support for posting a tweet longer than 140 characters as 8e9e812 .

As all of you know, the implementation is not so simple as replacing 140 with 280.
(You can see it at 8e9e812 .)
When I had read the entry https://blog.twitter.com/official/en_us/topics/product/2017/tweetingmadeeasier.html , I had failed to find the strict definition of

Japanese, Korean, and Chinese will continue to have 140 characters.

Which character should be counted by the same way as Japanese? In fact, some other characters such as the Ethiopic ones and Cherokee ones also counted as similarly to Japanese. To implement this, I need the strict definition.
Now, the definition still seems not to be provided as a public official REST API. I have taken a forcible method that directly retrieves the definition from https://github.com/twitter/twitter-text/ , which is committed at 2017-12-15. However, I think such a method may not be officially recommended.

[afukumoto]

In addition to 8e9e812, I think twittering-edit-length-check needs modification to update twittering-warning-overlay properly in accordance with weighted character length.

[cvmat]

Thank you, @afukumoto .
As you pointed out, some patches are required for fixing the warning overlay on the edit buffer.
I have committed them as c27d9b5 .
Please try it.

[afukumoto]

It works well, thank you!

[Wilfred]

#142 is now closed and fixed, so I think this PR can be closed too.