[FEATURE] Support setting 'projectAssets' in 'theme.json'

[canufeel]

This PR implements a change needed to alter custom theme assets folder name to something else.
The folder name itself in the input and output is mirrored so given public assets folder in the custom theme would result in having public assets folder in the compiled output with all the links inside files set properly.

To set the project assets folder one would need to have something like the following in his theme.json file:

{
    "yuiGridsUrl": "http://yui.yahooapis.com/3.9.1/build/cssgrids/cssgrids-min.css",
    "yuiSeedUrl": "http://yui.yahooapis.com/combo?3.9.1/build/yui/yui-min.js",
    "projectAssets": "public"
}

[yahoocla]

Thank you for submitting this pull request, however I do not see a valid CLA on file for you. Before we can merge this request please visit https://yahoocla.herokuapp.com/ and agree to the terms. Thanks! 😄

[yahoocla]

CLA is valid!