PR for v.1.2.1

[hcpchris]

• Closes BUG: Datetime serialized to SECONDS from Unix epoch rather than MILLISECONDS from Unix epoch #61
• Closes Enhancement: Auto-convert PlotLines.value from datetime to POSIX timestamps #58
• Closes plotLines example with datetime objects #57

[codecov]

Codecov Report

Merging #62 (4fe4116) into master (cc197bf) will decrease coverage by 0.40%.
The diff coverage is 62.13%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
- Coverage   90.41%   90.01%   -0.40%     
==========================================
  Files         206      206              
  Lines       27075    27491     +416     
  Branches     1749     1784      +35     
==========================================
+ Hits        24479    24746     +267     
- Misses       1902     2044     +142     
- Partials      694      701       +7     

Impacted Files	Coverage Δ
highcharts_core/options/axes/numeric.py	100.00% <ø> (ø)
highcharts_core/options/axes/plot_bands.py	98.93% <0.00%> (-1.07%)	⬇️
highcharts_core/options/plot_options/arcdiagram.py	100.00% <ø> (ø)
highcharts_core/options/plot_options/area.py	100.00% <ø> (ø)
highcharts_core/options/plot_options/bellcurve.py	100.00% <ø> (ø)
highcharts_core/options/plot_options/boxplot.py	92.90% <ø> (ø)
highcharts_core/options/plot_options/bubble.py	94.87% <ø> (ø)
highcharts_core/options/plot_options/bullet.py	94.33% <ø> (ø)
...harts_core/options/plot_options/dependencywheel.py	96.17% <ø> (ø)
highcharts_core/options/plot_options/dumbbell.py	96.37% <ø> (ø)
... and 109 more