Add set_speed method

[bdraco]

Closes #127

[codecov-commenter]

Codecov Report

Merging #128 (881bc39) into master (19d9de8) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
+ Coverage   87.43%   87.56%   +0.12%     
==========================================
  Files          12       12              
  Lines         788      796       +8     
==========================================
+ Hits          689      697       +8     
  Misses         99       99              

Impacted Files	Coverage Δ
pywizlight/bulb.py	90.05% <100.00%> (+0.20%)	⬆️

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[bdraco]

Looks like we need another release for this