feat: add start option to post build

[alexjfno1]

Fixes #146

Added

This PR adds the ability to specify a custom start command, wait-on & wait-on-timeout in the postBuild stage of the build. It mimics the behaviour of the options in the preBuild stage.

Example:

[plugins.inputs.postBuild]
    enable = true
    start = 'npx serve public'
    wait-on = 'http://localhost:5000'
    wait-on-timeout = '30' # seconds

It also contains refactoring of the index.js file to break it into smaller files as I felt it was getting too large. Jest has also been added for unit testing of these files.

Tasks

• Add the ability to specify a start option in inputs.postBuild which will run the command instead of serving the public folder. If no start command is specified the original behavior still exists.
• Add Jest to unit test functionality. This can be run with npm run test:unit.
• Added unit tests for onPostBuild.
• Add CircleCI test job
• Refactor index.js into smaller files.
• Update README

[CLAassistant]

All committers have signed the CLA.

[alexjfno1]

Hi @bahmutov,

This is a PR to solve #146, there are still a few things I need to do but what are your thoughts on this?

[alexjfno1]

@bahmutov do I need to configure something to get the build on CircleCI to work correctly?

[bahmutov]

It is pretty annoying, since it requires to use Netlify during tests, thus it needs private token, ughh. Let me merge your PR into my branch then all the tests will run and we will see.