Deprecate plot_graph and render_graph(), use different names

[wlandau]

Use vis_drake_graph() and render_drake_graph() instead. Lots of people plot and render graphs, so I thought these name changes would be particularly important for #147 and ropensci/software-review#156

[lintr-bot]

R/deprecate.R:517:25: style: Variable and function names should be all lowercase.

​    navigationButtons = navigationButtons,
                        ^~~~~~~~~~~~~~~~~

[codecov-io]

Codecov Report

Merging #151 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #151   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          57     57           
  Lines        3509   3593   +84     
=====================================
+ Hits         3509   3593   +84

Impacted Files	Coverage Δ
R/make.R	100% <ø> (ø)	⬆️
R/read.R	100% <ø> (ø)	⬆️
R/basic_example.R	100% <ø> (ø)	⬆️
R/color.R	100% <ø> (ø)	⬆️
R/workplan.R	100% <ø> (ø)	⬆️
R/parallel_ui.R	100% <ø> (ø)	⬆️
R/dataframes_graph.R	100% <ø> (ø)	⬆️
R/dataframes_graph_utils.R	100% <ø> (ø)	⬆️
R/outdated.R	100% <ø> (ø)	⬆️
R/deprecate.R	100% <100%> (ø)	⬆️
... and 7 more

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