Refactor asset type specifiers to use a class system, instead of tuples

For example, instead of an asset specification that looks like:

assets = {
    'main-less': ('/_eagle/css/main.less', 'less'),
    'main-js': ('/_eagle/js/theme.js', 'requirejs'),
}

go with something like:

assets = {
    'main-less': LessAsset('/_eagle/css/main.less'),
    'main-js': RequireJSAsset('/_eagle/js/theme.js'),
}

This has a couple of advantages:

Allows passing additional configuration arguments to the instance, such as less version, require.js config, etc. Some of these configuration options are currently theme-level, but they really should be asset-level.
Allows asset compilers to be built or subclassed outside of pyramid_frontend.
Eliminates the need for a lookup table during asset compilation and tag generation: can just call methods on the asset instances directly.

storborg / pyramid_frontend