Fix `—drop_blanks` for `filter_features` command line script

[desilinguist]

This PR closes #693.

• Raise an exception if —drop_blanks removes all rows in the input file if each of them contains a blank value.
• Add a test for this scenario.
• Update —help output for filter_features to make it clear that —drop_blanks and —replace_blanks_with only apply to CSV/TSV files.

[codecov[bot]]

Codecov Report

Merging #703 (3a697b6) into main (a5d5b3f) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #703   +/-   ##
=======================================
  Coverage   96.89%   96.89%           
=======================================
  Files          63       63           
  Lines        9197     9213   +16     
=======================================
+ Hits         8911     8927   +16     
  Misses        286      286           

Impacted Files	Coverage Δ
skll/utils/commandline/filter_features.py	98.48% <ø> (ø)
skll/data/readers.py	90.26% <100.00%> (+0.06%)	:arrow_up:
tests/test_commandline_utils.py	99.67% <100.00%> (+<0.01%)	:arrow_up:

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[desilinguist]

The code coverage issue can be ignored.