Refactor the CLI, part 5 (plus archive subcommand)

[fghaas]

• Replace subprocess.check_call() for creating a symlink with os.symlink()
• Re-implement subcommand functionality in a way that it can easily be called from invoke
• Move validation for course run names and dates into the new_run() method
• Add archive subcommand, which creates a gzipped tarball for upload into Open edX Studio

[codecov-io]

Codecov Report

Merging #36 into master will increase coverage by 0.47%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   99.17%   99.64%   +0.47%     
==========================================
  Files           9       11       +2     
  Lines         485      568      +83     
==========================================
+ Hits          481      566      +85     
+ Misses          4        2       -2

Impacted Files	Coverage Δ
tests/test_full_course.py	100% <100%> (ø)	:arrow_up:
olxutils/archive.py	100% <100%> (ø)
olxutils/cli.py	97.84% <100%> (+2.61%)	:arrow_up:
tests/tasks.py	100% <100%> (ø)

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