Rename `log` output option to `logs`

[desilinguist]

This PR closes #520.

• Modify configuration parser to allow 'logs' field in addition to log.
• Show a warning if the user uses 'log' instead of 'logs'.
• Update all of the examples configuration files to use 'logs'.
• Update all tests in test_input.py to use 'logs'.
• Add a new test in test_input.py.
• Update the tutorial and the documentation.

[codecov[bot]]

Codecov Report

Merging #670 (d97c4af) into main (c339845) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #670   +/-   ##
=======================================
  Coverage   96.76%   96.76%           
=======================================
  Files          63       63           
  Lines        9077     9098   +21     
=======================================
+ Hits         8783     8804   +21     
  Misses        294      294           

Impacted Files	Coverage Δ
skll/config/__init__.py	95.28% <100.00%> (+0.17%)	:arrow_up:
tests/test_input.py	99.80% <100.00%> (+<0.01%)	:arrow_up:
tests/utils.py	97.89% <100.00%> (ø)

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