Add the ability to run polkadot using a config file

[s3krit]

The polkadot binary currently supports 84 unique command-line flags for controlling its operation (some mutually exclusive, some not). For users with larger lists of flags to invoke the binary with, this could be better served by allowing them to run using a config-file. We could ship the binary with a default config file which could fairly verbosely explain (via comments) the impact and intent, and examples of these options, without needing a 381-line output of polkadot --help.

Questions:

• Should CLI flags be moved to a config file format, or simply copied and flags take precedence over config file directives
• What format should we use for the configuration file? (iirc we use clap for the flags, so maybe that already supports some method of transposing these to a config file format)
• If we move certain directions to being controlled by a config file rather than CLI flags, this would constitute a breaking change for a lot of node operators, so how should we handle this?