Add ability to return a "features" list in the welcome message.

This could help users quickly discover the presence of an API or mode of
operation.

Features are obtained from a new config API `config:features/0`

System components can enable features via `config:enable_feature/1` API, or
users can directly enable features in the .ini files, and those features
should show up in the "Welcome" message.

COUCHDB-3180