FlexSlider - enable Thumbnail navigation

[jsirish]

closes #1

[jsirish]

closes #83

[codecov]

Codecov Report

Merging #85 into 1.0 will increase coverage by 0.14%.
The diff coverage is 59.25%.

@@            Coverage Diff             @@
##              1.0      #85      +/-   ##
==========================================
+ Coverage   69.42%   69.56%   +0.14%     
==========================================
  Files           2        2              
  Lines         121      161      +40     
==========================================
+ Hits           84      112      +28     
- Misses         37       49      +12

Impacted Files	Coverage Δ
code/FlexSlider.php	56.63% <59.25%> (+7.32%)	✅

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[jsirish]

@muskie9 this should be ready to go. Ran into an issue with adding all the new config options in CMS where existing installs with thumbnails will be wonky without the three carousel thumbnail settings. To fix, added populate defaults, an onBeforeWrite to set defaults if empty, and a task to set the "new" defaults on existing FlexSlider instances.

Might be overkill since there are no existing installations with thumbnails, but thought I'd point it out