added filter "requirejs" for AMD-style javascript modularization

this filter adds support for the "r.js" external tool, the node-based RequireJS javascript modularization and optimization "compiler". it directly supports the following "r.js" options:

* file-based JSON configuration: via ``env.config['requirejs_config']``
* *optimize*: via ``env.config['requirejs_optimize']``
* *baseUrl*: via ``env.config['requirejs_baseUrl']``

any other "r.js" options can be specified via ``env.config['requirejs_extras']``.

other webassets-specific config options (such as ``requirejs_run_in_debug``) are documented in the filter's pydocs.