better loadNpmTask injection

I managed to add string arrays as an input for loadNpmTask; I also modified the existing test and README
So we want an array as an input? The line
```
pluginName = ( _.isString(pluginName) && pluginName.trim() ) || (_.isArray(pluginName) && pluginName.length ) || false;
```
Is there to allow strings or array of strings to be an input, should we drop single string support?