Allow for greater configuration of this plugin's internal Rollup build

[shalomscott]

Overview

I thought it would be neat to allow users of this plugin to configure the internal Rollup build as much as possible. At the same time, I don't want to add individual configuration properties for each and every Rollup option there is. What better way to enable extensibility than through an actual rollup.config.js file?

This PR does the following:

• Removes the rollupInputPlugins and rollupOutputPlugins plugin options
• If present, reads the rollup.config.js file and uses it (as much as possible) when building Svelte components

Still haven't figured out:

• How to remove the rollupPluginSvelteOptions option as well and enable configuration of rollup-plugin-svelte in the rollup.config.js file.