Implement support for ODBC-style and other database-specific connection strings

[dossy]

It would be nice for dbmate to support specifying ODBC-style connection strings (and, database-driver specific connection string formats) rather than a fully-formed database URL, as it may be easier to hand-write the connection string rather than having to URI-encode characters in the URL by hand.

See #435 for some background on this.

I think adding the CLI argument --connection-string value and parsing the ODBC-style connection string and transforming it into a well-formed database URL, which dbmate can use, can be useful functionality for some dbmate users.

The difficulty here is that, unfortunately, PostgreSQL connection strings do not follow the ODBC specification for connection strings. 😞

This means having a single connection string parsing implementation won't be possible, and it'll need to be per-driver, and we'll need something like a second CLI argument --driver value to specify which driver the connection string is intended for.

[gregwebs]

Maybe the main problem here is just the password. One thing that worries me about all this from the code is:

// redactLogString attempts to redact passwords from errors

A separate mechanism for passing the password could be used to improve security and usability.

[gregwebs]

A validated uri requires certain fields be filled out. But It would be easier for me to use dbmate if I could just use the PG* environment variables like I do elsewhere.