Improved course archive support

[fghaas]

• Add 3 more directories to the tarball created by olx archive.
• Add a -b/--base-name option to olx archive.
• Add Python 3.8 to the tox/travis test matrix.

[codecov-commenter]

Codecov Report

Merging #41 into master will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   97.81%   97.83%   +0.01%     
==========================================
  Files          15       15              
  Lines         870      876       +6     
==========================================
+ Hits          851      857       +6     
  Misses         19       19              

Impacted Files	Coverage Δ
olxutils/archive.py	97.14% <ø> (ø)
olxutils/cli.py	94.52% <100.00%> (ø)
tests/tasks.py	100.00% <0.00%> (ø)
tests/test_cli.py	100.00% <0.00%> (ø)
olxutils/helpers.py	100.00% <0.00%> (ø)

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