right-align labels in showvalues (not left-align)

[stevengj]

Before this PR, labels in showvalues are left-aligned, which looks weird:

julia> x,n = 1,10
(1, 10)

julia> p = Progress(n);

julia> for iter in 1:10
           x *= 2
           sleep(0.5)
           next!(p; showvalues = [("iteration count",iter), ("x",x)])
       end
Progress: 100%|█████████████████████████████████████████| Time: 0:00:05
  iteration count:  10
  x:                1024

afterwards, they are right-aligned:

julia> for iter in 1:10
           x *= 2
           sleep(0.5)
           next!(p; showvalues = [("iteration count",iter), ("x",x)])
       end
Progress: 100%|█████████████████████████████████████████| Time: 0:00:05
  iteration count:  10
                x: 1024

which seems more readable to me.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.73%. Comparing base (807496a) to head (4558a73).
Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #330      +/-   ##
==========================================
+ Coverage   93.48%   96.73%   +3.25%     
==========================================
  Files           1        1              
  Lines         399      552     +153     
==========================================
+ Hits          373      534     +161     
+ Misses         26       18       -8     

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