Fix paragraph handling

[quantsch]

Paragraphs are not handled as they should:

test_that("Paragraphs are handled correctly", {
  expect_equal(rd_str_to_md("line-one\nline-two"), "line-one\nline-two\n")

  # TODO: these tests fail, fix behaviour:
  expect_equal(
    rd_str_to_md("paragraph-one\n\nparagraph-two"),
    "paragraph-one\n\nparagraph-two\n"
  )
  expect_equal(
    rd_str_to_md("paragraph-one\n\n\nparagraph-two"),
    "paragraph-one\n\nparagraph-two\n"
  )
})