Add plugin top-level configuration

[teresam856]

Proposal for issue 1535

Adds method to PluginManager that retrieves plugin's configuration objects if they exist
Modifies the Jbrowse Model to add the plugin's configurations
Adds optional configuration param to the base class for a JBrowse plugin
Adds a test and a test plugin to verify we are able to read and get from the configuration

[codecov]

Codecov Report

Merging #1711 (0cffadc) into master (dd44843) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1711      +/-   ##
==========================================
+ Coverage   58.93%   58.95%   +0.02%     
==========================================
  Files         446      447       +1     
  Lines       20461    20471      +10     
  Branches     4818     4819       +1     
==========================================
+ Hits        12058    12068      +10     
  Misses       8105     8105              
  Partials      298      298              

Impacted Files	Coverage Δ
products/jbrowse-web/src/jbrowseModel.js	65.85% <ø> (ø)
packages/core/Plugin.ts	100.00% <100.00%> (+50.00%)	⬆️
packages/core/PluginManager.ts	94.11% <100.00%> (+0.17%)	⬆️
products/jbrowse-web/src/tests/TestPlugin.ts	100.00% <100.00%> (ø)
packages/core/util/analytics.ts	83.78% <0.00%> (-2.71%)	⬇️
packages/core/util/index.ts	81.87% <0.00%> (ø)

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[rbuels]

this is very elegant 👍

just one little rename needed I think

[rbuels]

looks great!