[MRG] Add --force flag to 'sourmash index'

[ctb]

This lets us do

sourmash index genbank-k31 ../.sbt.genbank-k31 --traverse-directory -f

to reindex SBTs, etc. etc.

• 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 #446 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #446      +/-   ##
==========================================
+ Coverage   90.43%   90.48%   +0.04%     
==========================================
  Files          33       33              
  Lines        4882     4886       +4     
  Branches       36       36              
==========================================
+ Hits         4415     4421       +6     
+ Misses        466      464       -2     
  Partials        1        1

Impacted Files	Coverage Δ
sourmash/sourmash_args.py	95.31% <100%> (ø)	⬆️
sourmash/commands.py	91.18% <100%> (+0.33%)	⬆️

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[ctb]

Ready for review!