Make @showprogress support keyword arguments

[dhanak]

This allows constructs such as:

@showprogress "X: " color=:red for x in 1:10
    sleep(1)
end

and

@showprogress showspeed=true for x in 1:10
    sleep(1)
end

Fixes #229.

[kleinschmidt]

Thanks for doing this! has been on my mind for a while 😁

It also (I think) should allow run-time disabling of progress with enabled=false kwarg.

I do think it needs tests to make sure that there aren't any wacky edge cases with for instance kwargs and positional args interleaved.

[kleinschmidt]

It also (I think) should allow run-time disabling of progress with enabled=false kwarg.

Ha I see now that that was the original feature request!

[dhanak]

I added some tests.

[timholy]

Somehow the tests didn't run, I'll close and reopen.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +1.03% 🎉

Comparison is base (9ebe384) 95.53% compared to head (5477723) 96.56%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
+ Coverage   95.53%   96.56%   +1.03%     
==========================================
  Files           1        1              
  Lines         515      524       +9     
==========================================
+ Hits          492      506      +14     
+ Misses         23       18       -5     

Files Changed	Coverage Δ
src/ProgressMeter.jl	96.56% <100.00%> (+1.03%)	⬆️

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