Allow users to fetch previous vintage of time series

[MattCowgill]

This adds a release_date argument to read_abs().

At the moment, read_abs() fetches the latest release of the requested table(s). It cannot be used to fetch previous releases.

The release_date argument allows previous releases to be retrieved. Note that it does not work for anything released prior to the launch of the current ABS website (mid-2019).

Keen for input on this PR.

[codecov]

Codecov Report

Base: 87.32% // Head: 87.72% // Increases project coverage by +0.40% 🎉

Coverage data is based on head (d67841e) compared to base (642730b).
Patch coverage: 91.17% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
+ Coverage   87.32%   87.72%   +0.40%     
==========================================
  Files          28       28              
  Lines        1049     1108      +59     
==========================================
+ Hits          916      972      +56     
- Misses        133      136       +3     

Impacted Files	Coverage Δ
R/search_files.R	0.00% <0.00%> (ø)
R/search_catalogues.R	76.19% <33.33%> (+16.19%)	⬆️
R/check_abs_connection.R	34.61% <46.15%> (-1.39%)	⬇️
R/read_awe.R	97.97% <96.00%> (-0.60%)	⬇️
R/read_abs.R	90.90% <96.15%> (+0.64%)	⬆️
R/check_latest_date.R	100.00% <100.00%> (ø)
R/download_abs.R	94.11% <100.00%> (-5.89%)	⬇️
R/download_data_cube.r	88.23% <100.00%> (+0.73%)	⬆️
R/extract_abs_sheets.R	86.36% <100.00%> (+0.64%)	⬆️
R/get_abs_xml_metadata.R	91.80% <100.00%> (+0.13%)	⬆️
... and 8 more

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[MattCowgill]

Three options:

1. do not add the release_date arg, so read_abs() can still only fetch the latest release;
2. merge this PR, so the release_date arg is added, but pre-2019 releases cannot be retrieved; or
3. close this PR, and instead write new logic that will be able to fetch pre-2019 releases (this would be fiddly as hell)

[MattCowgill]

Went with option (2)