Support tidy evaluation (solve #200)

[wlandau]

@krlmlr, you may be interested as well: this PR adds optional tidy evaluation to drake_plan().

[codecov-io]

Codecov Report

Merging #202 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #202   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          61     61           
  Lines        4282   4292   +10     
=====================================
+ Hits         4282   4292   +10

Impacted Files	Coverage Δ
R/make.R	100% <ø> (ø)	⬆️
R/build.R	100% <100%> (ø)	⬆️
R/workplan.R	100% <100%> (ø)	⬆️
R/meta.R	100% <100%> (ø)	⬆️
R/migrate.R	100% <100%> (ø)	⬆️
R/dependencies.R	100% <100%> (ø)	⬆️

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