connect task refactoring

- changed task to only contain "openui5_connect" specific options.
"connect" options have to be configured in a separate "connet" target
with the same name (see README docs)
- added proxy middleware
- bump "connect-openui5" dependency to latest version

fixes https://github.com/SAP/grunt-openui5/issues/8
closes https://github.com/SAP/grunt-openui5/pull/7