Add build step

[micaww]

Reverts the previous merge that removes destructuring usages in favor of a build step similar to clay-kiln, in order to ensure ES5 output compatible with all browsers.

Adds the following npm scripts:

• npm run prepublishOnly for a production build before publishing to npm
• npm run build or npm run watch for a development build

Only the output bundle will be published to npm, rather than the source/test suite.

[coveralls]

Coverage remained the same at 100.0% when pulling 367a79b on Entercom:add-build-step into 99791ff on clay:master.

[olsonpm]

do we want last 2 versions ? I'd assume you'd want browserslist's "defaults"

https://github.com/browserslist/browserslist#best-practices

Edit: also the babel docs have a note by targets.browsers

https://babeljs.io/docs/en/babel-preset-env#targetsbrowsers

Note: this will be removed in later version in favor of just setting "targets" to a query directly.

so I think instead you'd just want "targets": <browserslist query>

https://babeljs.io/docs/en/babel-preset-env#targets

[micaww]

last 2 versions only because that's what clay-kiln uses. defaults might be better anyways since it's more broad.

[olsonpm]

ah - I didn't realize that was in sync with clay-kiln. Using whatever clay-kiln has is probably best then to keep things consistent :)

[olsonpm]

or defaults haha - maybe the clay team will have an opinion

[pedro-rosario]

Anything that supports the version clay-kiln uses is fine. defaults sounds like a good choice.