[MRG] clear to eol in notify(...) and error(...)

[ctb]

Fixes #344 by using magic vt100 commands to clear to EOL before printing.

Also cleans up sourmash compute --merge output for multiple signatures - fixes #352.

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov-io]

Codecov Report

Merging #351 into master will increase coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
+ Coverage   90.29%   90.49%   +0.19%     
==========================================
  Files          33       33              
  Lines        4885     4891       +6     
  Branches       36       36              
==========================================
+ Hits         4411     4426      +15     
+ Misses        473      464       -9     
  Partials        1        1

Impacted Files	Coverage Δ
sourmash/commands.py	91.21% <100%> (+0.03%)	⬆️
sourmash/logging.py	100% <100%> (ø)	⬆️
sourmash/sbt_storage.py	95.5% <0%> (-1.09%)	⬇️
sourmash/lca/lca_utils.py	94.47% <0%> (+1%)	⬆️
sourmash/signature_json.py	95.51% <0%> (+1.92%)	⬆️
tests/test_signature_json.py	100% <0%> (+7.57%)	⬆️

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[ctb]

Ready for review!