Location of JS/script assets in the HTML

The user can include additional JS scripts in the HTML page by passing the URLs via the assets keyword to HTML. Currently, the generated <script> tags get placed after the built-in dependencies (including RequireJS, which can cause issues). Based on that discussion, it looks like we could handle it better:

• By default, place the user-provided <script> tags before the RequireJS loading (works around the issues with libraries that do not play well with RequireJS if they are loaded separately: https://github.com/requirejs/requirejs/issues/947#issuecomment-28677913).
• Allow the user to customize the location with a keyword to asset() (e.g. before, after RequireJS, at the end of the HTML file).