CRAN hotfix

• In R 3.5.0, the !! operator from tidyeval and rlang is parsed differently than in R <= 3.4.4. This change broke one of the tests in tests/testthat/tidy-eval.R The main purpose of drake's 5.1.2 release is to fix the broken test.
• Fix an elusive R CMD check error from building the pdf manual with LaTeX.
• In drake_plan(), allow users to customize target-level columns using target() inside the commands.
• Add a new bind_plans() function to concatenate the rows of drake plans and then sanitize the aggregate plan.
• Add an optional session argument to tell make() to build targets in a separate, isolated master R session. For example, make(session = callr::r_vanilla).