v0.2.0

 * Major change - use pg_settings to retrieve runtime variables. Adds >180
   new metrics and descriptions (big thanks to Matt Bostock for this work).

   Removes the following metrics:
   ```
   pg_runtime_variable_max_connections
   pg_runtime_variable_max_files_per_process
   pg_runtime_variable_max_function_args
   pg_runtime_variable_max_identifier_length
   pg_runtime_variable_max_index_keys
   pg_runtime_variable_max_locks_per_transaction
   pg_runtime_variable_max_pred_locks_per_transaction
   pg_runtime_variable_max_prepared_transactions
   pg_runtime_variable_max_standby_archive_delay_milliseconds
   pg_runtime_variable_max_standby_streaming_delay_milliseconds
   pg_runtime_variable_max_wal_senders
   ```

   They are replaced by equivalent names under `pg_settings` with the exception of
   ```
   pg_runtime_variable_max_standby_archive_delay_milliseconds
   pg_runtime_variable_max_standby_streaming_delay_milliseconds
   ```
   which are replaced with
   ```
   pg_settings_max_standby_archive_delay_seconds
   pg_settings_max_standby_streaming_delay_seconds
   ```