config.templatesFn is optional, so should use an empty object by default

The addition of the `config.templatesFn` option should be optional, but the `defaultConfig` does not specify it, causing an error at https://github.com/rrharvey/grunt-file-blocks/blob/d3e546b65df001ca6125237b8911f341b375771b/lib/fileprocessor.js#L90 when accessing the property.