Add sorted options for Plotly

[magic7s]

I found the output gantt to be sorted (in reverse direction) by the way the tasks were entered. Adding this to the plotly output was fairly straightforward. I'm trying to decipher how to add it to matplotlib output as well. I thought I would ship this as it is working and matplotlib can be addressed in a subsequent PR.

[codecov-commenter]

Codecov Report

Merging #45 (f519bb7) into master (9fffa1c) will decrease coverage by 0.02%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   94.86%   94.84%   -0.03%     
==========================================
  Files          30       30              
  Lines        3270     3276       +6     
==========================================
+ Hits         3102     3107       +5     
- Misses        168      169       +1     

Impacted Files	Coverage Δ
test/test_gantt.py	99.23% <ø> (ø)
processscheduler/solution.py	94.92% <83.33%> (-0.37%)	:arrow_down:

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[magic7s]

Rebased