parse_config_file: alternate comment interpretation

For some time `parse_config_file` treats all `#` signs as initiating comments, preventing the use of `#` signs in values.

It would be very helpful if a simple flag could be set, to only consider # signs in column 0, or preceded only by whitespace, as comments. This would make for clearer, cleaner code and configuration files, when configuration values contain `#`.

Here's an example of somebody looking for this, and being advised to write an iostreams filter to make it work:
https://stackoverflow.com/questions/31921241/boostprogram-options-how-to-support-hash-character-in-a-value

Here's an example of brute-force key generation implemented in a production release, to make sure the key never contains the `#` symbol and can be stored in the configuration file:
https://github.com/libbitcoin/libbitcoin-protocol/blob/version3/src/zmq/certificate.cpp#L78

Start by locating the parse_config_file implementation and its existing tests or documentation. Review how # is currently interpreted, then determine how an opt-in setting should treat # in column 0 or after whitespace while preserving # in values. Done means the requested setting is documented and covered by tests for both comment and value cases.

Written by the indexing model from the issue text.