Add silent parameter to get_stats19

[agila5]

As we discussed in #140, I added a silent parameter to get_stats19. For example:

res <- stats19::get_stats19(2018)
#> Files identified: dftRoadSafetyData_Accidents_2018.csv
#>    http://data.dft.gov.uk.s3.amazonaws.com/road-accidents-safety-data/dftRoadSafetyData_Accidents_2018.csv
#> Attempt downloading from:
#> Data saved at /tmp/Rtmp9K14gT/dftRoadSafetyData_Accidents_2018.csv
#> Reading in:
#> /tmp/Rtmp9K14gT/dftRoadSafetyData_Accidents_2018.csv
#> date and time columns present, creating formatted datetime column

^{Created on 2020-01-29 by the reprex package (v0.3.0)}

and

res <- stats19::get_stats19(2018, silent = TRUE)
#> date and time columns present, creating formatted datetime column

^{Created on 2020-01-29 by the reprex package (v0.3.0)}

I choose not to silent in any case the new date/time message but it's really easy to add that.

[codecov-io]

Codecov Report

Merging #149 into master will increase coverage by 0.41%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage    69.6%   70.02%   +0.41%     
==========================================
  Files           7        7              
  Lines         408      417       +9     
==========================================
+ Hits          284      292       +8     
- Misses        124      125       +1

Impacted Files	Coverage Δ
R/get.R	97.56% <100%> (+0.12%)	⬆️
R/read.R	98.48% <100%> (+0.02%)	⬆️
R/dl.R	80.85% <91.66%> (+0.36%)	⬆️

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[Robinlovelace]

Interesting, only just realised there is an 'isFALSE()` function in base R, more elegant that !silent!