Integrate all previous PRs

[k3a]

I integrated all open PRs into my fork:

• Add support for uppercase encodings #42 MERGED make encoding case-insensitive
• Check for attachments before jumping into switch/case statement #41 MERGED to allow attaching text/html attachments
• fix(content-type-encoding): Added support for quoted printable content type encoding #39 MERGED (test case only as quoted-printable implemented by another PR) adds quoted-printable
• fix: inner mime type quoted-printable #38 MERGED use NextRawPart instead of NextPart and decode content the same way as the rest of the library
• Add 8bit and binary decoding support #37 MERGED TODO NEEDS MANUAL FIX TO AVOID RE-ENCODE Add 8bit and binary decoding support
• Add support binary encoding #31 SKIPPED incomplete duplicate of 37
• Added parsing attachments in non-multipart emails #29 MERGED Parse email with just a binary attachment and no text
• converts encoding to lowercase #28 SKIPPED duplicate of 42
• handle lower encoding, add quoteprintable, fix attachment handling, f… #26 MERGED Decode content with supported Content-Encoding everywhere
• Find Attachments, dont matter the content-type msg #25 SKIPPED duplicate of 26
• Support to Encapsulated Messages (message/rfc822) inside multipart/mixed #27 TODO: Support to Encapsulated Messages (message/rfc822) inside multipart/mixed
• Decode text/plain and text/html body with encoding #24 MERGED (test case for base64 encoding only)

Some of them required manual merging but the resulting code should be correct, hopefully.

I am providing this as a PR to the original repo first.
The library is quite nice but the original repo seems to be unmaintained for more than 3 years and the library is largely broken without those PRs.

If there is no response by the original author in a week, I will attempt to make my repo to be some kind of 'official fork', set up CI for it and handle any future PRs.