implement geom_errors() providing automatic errorbars

[Enchufa2]

Following the ideas in r-quantities/units#294

[codecov]

Codecov Report

Merging #52 (6b20f8f) into master (9721457) will decrease coverage by 1.02%.
The diff coverage is 97.40%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   96.86%   95.84%   -1.03%     
==========================================
  Files          10       12       +2     
  Lines         383      481      +98     
==========================================
+ Hits          371      461      +90     
- Misses         12       20       +8     

Impacted Files	Coverage Δ
R/tidyverse.R	94.87% <ø> (ø)
R/plot.R	72.00% <75.00%> (ø)
R/geom_errors.R	98.63% <98.63%> (ø)

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