Support custom connector descriptions

[misolt]

Allow for more customization of how help text looks by connector developers. For example:

* snowflake-information-schema-metadata - Dumps metadata from Snowflake, using INFORMATION_SCHEMA only.
        [same options as 'snowflake']
        -Dsnowflake.metadata.databases.query=value              Custom query for databases dump.
        -Dsnowflake.metadata.databases.where=value              Custom where condition to append to query for databases dump.
        -Dsnowflake.metadata.schemata.query=value               Custom query for schemata dump.

Before this PR, the full list of options would be provided for each connector. The customization is done by overriding the new method Connector#printHelp.