Changes to themes/load! plugin preparing for delite/deliteful split.

- Make plugin take arbitrary paths.  Paths should contain {{theme}} substitution variable.
- Remove RTL handling code from load!, punting the job to the widgets.
- Allow for other projects to "subclass" load.js w/a custom themeMap etc.

A widget with both LTR and RTL CSS will have a define() dependency list including entries like:

    "./themes/load!./themes/{{theme}}/StarRating",
    "dojo/has!dojo-bidi?./themes/load!./themes/{{theme}}/StarRating_rtl"

The order the CSS is inserted into the document is not guaranteed, so the selectors in
StarRating_rtl should override the selectors in StarRating.

Refs #14, fixes #53.