Fix Hmisc label issues

tableone 0.11.2 (2020-07-13)

BUG FIXES by @ndevln (PR #66)

• Hmisc labels no longer cause continuous vars to be treated as
  factors (Closes Issue #64)

• Setting value labels via labelled no longer result in tableone
  dropping variables.

ADDITIONAL BUG FIXES

• Add rmarkdown to Suggests.