Build up a hash of disabled plugin instances, keyed by alias, with the value being whether it's disabled or not
This hash will contain an entry for every installed plugin.

If a plugin has not been referenced in `config.pipeline.disabled` then it will be added to the map as `disabled: false`.

Example return value:

{
foo: true,
bar: false
}
function(aliasMap, config) {
    var aliases = this._flattenAliasMap(aliasMap);
    var allExcept = null;

    if (typeof config.allExcept !== 'undefined') {
      allExcept = this._castArray(config.allExcept);
      delete config.allExcept;
    }

    var keys = Object.keys(config);

    if (allExcept) {
      keys = keys.concat(allExcept);
    }

    var unknownConfigKeys = _.difference(keys, aliases);

    if (unknownConfigKeys.length) {
      this._logUnknownPlugins(unknownConfigKeys, 'config.pipeline.disabled');
    }

    var disabledMap = aliases.reduce(function(map, alias) {
      if (map[alias] === undefined) {
        if (allExcept && allExcept.length) {
          if (allExcept.indexOf(alias) >= 0) {
            map[alias] = false;
          } else {
            map[alias] = true;
          }
        } else {
          map[alias] = false;
        }
      }

      return map;
    }, config);

    return disabledMap;
  }