Allow extraParams to be added to the store requests

[geographika]

Allows additional parameters to always be sent with store requests (for example to support MapServer runtime-substitution variables:

    extraParams: {
        someParam: 'someValue'
    }

For example:

Ext.define('CpsiMapview.view.grids.MyLayerStore', {
    extend: 'CpsiMapview.store.WfsFeatures',
    url: '/mapserver/?',
    alias: 'store.MyLayer',
    storeId: 'MyLayer',
    model: 'CpsiMapview.view.grids.MyLayerModel',
    typeName: 'MyLayer',
    layerOptions: {
        displayInLayerSwitcher: false,
        name: 'MyLayer'
    },
    extraParams: {
        someParam: 'someValue'
    }
});