Add support for sslmode option for postgres params in cfg and cli.

[maroshmka]

config file

[target]
host = localhost
port = 5432
dbname = users
user = postgres
sslmode = require

CLI option - pgquarrel -c my.cfg --source-sslmode=require

more on sslmode - https://www.postgresql.org/docs/9.1/libpq-connect.html

[eradman]

My vote would be to allow pgquarrel to accept URLs instead

--source postgresql://... --target postgresql://...

This would allow arbitrary options that libpq supports

[maroshmka]

🤔 thats good idea, maybe there could be source.uri i do not have much time for it, but i might take a look and change it

[eulerto]

@maroshmka @eradman I don't want to add another command line option if libpq supports a way to pass a connection string. See https://github.com/eulerto/pgquarrel/issues/76#issuecomment-555769487

[eulerto]

Since this feature is already supported, I'm closing this PR.