Fix overprinting in ProgressUnknown

[palday]

This just adds in the same logic that's present in ProgressThresh and Progress.

[IanButterworth]

@palday I don't use IJulia so wouldn't know how to test this, and tests don't cover this.
If you could add some examples of before and after, that would be helpful

[codecov]

Codecov Report

Merging #204 (fb2963f) into master (aae026e) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
+ Coverage   96.32%   96.34%   +0.02%     
==========================================
  Files           1        1              
  Lines         490      493       +3     
==========================================
+ Hits          472      475       +3     
  Misses         18       18              

Impacted Files	Coverage Δ
src/ProgressMeter.jl	96.34% <100.00%> (+0.02%)	⬆️

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