Remove fixtures and use Cypress for integration tests

[mwistrand]

• Remove the fixtures and update the functional tests use Cypress
• Update the test-app build to copy an index.html file that loads the build custom elements
• Clean up the .gitignore
• Add integration tests for the --legacy flag
• Improve features logic

Resolves #46

[codecov[bot]]

Codecov Report

Merging #48 into master will increase coverage by 0.08%. The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   69.83%   69.91%   +0.08%     
==========================================
  Files           8        8              
  Lines         358      359       +1     
  Branches       62       62              
==========================================
+ Hits          250      251       +1     
  Misses        108      108

Impacted Files	Coverage Δ
src/base.config.ts	31.42% <50%> (+0.99%)	:arrow_up:

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[agubler]

@mwistrand looks like you have a failure on appveyor