Fixed malformed tables

[mitchelloharawild]

• Tables with tailing TEXT (such as \n) now do not contain extra last rows
• Empty separator detection is row robust to other separators (such as \code{})

Resolves #928

[jayhesselberth]

Thanks! Could you please write a unit test for the changes? I think this needs to pass:

test_that("code blocks in tables render (#978)", {
  expect_equal(
    rd2html('\\tabular{ll}{a \\tab \\code{b} \\cr foo \\tab bar}')[[2]],
    "<tr><td>a</td><td><code>b</code></td></tr>"
  )
})