Interim development release: proper clustermq support and memoized preprocessing

• Add a proper clustermq-based parallel backend: make(parallelism = "clustermq").
• Smooth the edges in vis_drake_graph() and render_drake_graph().
• Make hover text slightly more readable in in vis_drake_graph() and render_drake_graph().
• Align hover text properly in vis_drake_graph() using the "title" node column.
• Optionally collapse nodes into clusters with vis_drake_graph(collapse = TRUE).
• Improve dependency_profile() show major trigger hashes side-by-side
  to tell the user if the command, a dependency, an input file, or an ouptut file changed since the last make().
• Choose more appropriate places to check that the txtq package is installed.
• Expose the template argument of clustermq functions (e.g. Q() and workers()) as an argument of make() and drake_config().
• Improve the help files of loadd() and readd(), giving specific usage guidance in prose.
• Bugfix: loadd(not_a_target) no longer loads every target in the cache.
• Bugfix: exclude each target from its own dependency metadata in the "deps" igraph vertex attribute (fixes #503).
• Add a new code_to_plan() function to turn R scripts and R Markdown reports into workflow plan data frames.
• Add a new drake_plan_source() function, which generates lines of code for a drake_plan() call. This drake_plan() call produces the plan passed to drake_plan_source(). The main purpose is visual inspection (we even have syntax highlighting via prettycode) but users may also save the output to a script file for the sake of reproducibility or simple reference.
• Memoize all the steps of build_drake_graph() and print to the console the ones that execute.