Add test to get https://xpaste.pro/p/t4q0Lsp0 as .txt file by default

[konard]

We should have test that converts https://xpaste.pro/p/t4q0Lsp0 url to download to .txt file.

We also should support forced conversion to markdown document (with same content as https://xpaste.pro/p/t4q0Lsp0) and if file is not making markdown document larger than 1500 lines, we should embed text file as text block inside markdown version of this page.

If it is larger than 1500 lines (or will make markdown larger than that) we should make zip archive with index.md and xpaste-pro-t4q0Lsp0.txt file, that will be just linked in the markdown document.