Add a priority parameter to ensure correct loading order

This adds a `priority` parameter to the query that allows you to specify the priority of a stylesheet.

My use case for this is that I have a "theme" stylesheet that must be applied after the default styles in the app. The theme stylesheet must be defined in `head` after all the default styles, to allow theme styles to overwrite default styles. Due to dynamic loading, sometimes the default styles are `require`d after the theme is.

This change lets you specify a priority for a stylesheet. A stylesheet with a higher priority will always be loaded into `head` after all lower-priority stylesheets. The default priority is 0.