open flag will open default browser

[purplecabbage]

Description

Use --open flag to open default browser after successful run/deploy

Related Issue

Closes #157

Motivation and Context

using '--open' is consistent with other tools

parcel serve --open
ng serve --open
webpack-dev-server --open 
npx http-server [path] --open
...

• https://parceljs.org/cli.html
• https://angular.io/cli/serve
• https://webpack.js.org/configuration/dev-server/
• https://www.npmjs.com/package/http-server

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #247 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #247   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines          766       773    +7     
  Branches       104       106    +2     
=========================================
+ Hits           766       773    +7     

Impacted Files	Coverage Δ
src/commands/app/deploy.js	100.00% <100.00%> (ø)
src/commands/app/run.js	100.00% <100.00%> (ø)

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