feat: recognise all-caps strings in the subject

[revelt]

fixes #102 — if subject's second character is a letter and it's an uppercase letter, NOW first character is not upper-cased, no matter the setting.

I've got bitten by this few times, when mentioning "HTML" or "XML" or "CSS" in the subject :)

Please review, feel free to edit. I added tests.

Thank you.