Add configuration file to Dumpling

## Enhancement
Most other utilities have a possibility to use a configuration file, instead of having to use command line arguments for all the different configuration options.

It also would allow to avoid using the MySQL password on the command line for dumpling, which is not considered to be safe!

Since both `-f` and `-c` is already used, I propose `--config` which is consistent with other tools, like tidb-lightning and tidb-server (`-config`).