Add -c|--limit-creds option to autopwn.

[mgerst]

The command filters all credentials without the given username(s). The option may be repeated as much as needed.

[codecov[bot]]

Codecov Report

Merging #23 into master will decrease coverage by 0.06%. The diff coverage is 68.96%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
- Coverage   70.34%   70.28%   -0.07%     
==========================================
  Files          19       20       +1     
  Lines        1150     1161      +11     
==========================================
+ Hits          809      816       +7     
- Misses        341      345       +4

Impacted Files	Coverage Δ
flag_slurper/autolib/models.py	100% <ø> (ø)	:arrow_up:
flag_slurper/autolib/service.py	100% <100%> (ø)	:arrow_up:
flag_slurper/autolib/utils.py	100% <100%> (ø)
flag_slurper/autolib/exploit.py	38.15% <100%> (+0.82%)	:arrow_up:
flag_slurper/autopwn.py	20.86% <27.27%> (+0.68%)	:arrow_up:
flag_slurper/autolib/protocols.py	20% <83.33%> (ø)	:arrow_up:

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[mgerst]

Closes #20