allow using unix domain socket paths in connection form -> server field

[eug3nix]

allow empty server valued in the connection form for postgres connections
add unix socket support for mariadb/mysql
refs: #437

test cases:
postgres connections should allow hostnames, IP addresses, empty values and absolute unix paths
mysql/mariadb connections should allow hostnames, IP addresses and absolute unix paths

postgres connections should look for sockets under default postgres socket path if server field is empty
postgres connections should look for sockets under the base path provided in the server field
mariadb/mysql connections should look for socket file specified by absolute socket path

other connection types: shoud work as before, file paths or empty values are not allowed
postgres/maridadb/mysql should work with ips and hostnames as before

[justinclift]

Cool, that looks like it's headed in the right direction. Do you reckon updating the associated help text for the field to mention unix sockets (etc) would be useful too? 😄

[eug3nix]

@justinclift I've added hints in server input placeholders per your suggestion.
These changes will be included in 1.1.1 version which we are planning to release soon.
Thanks!

[justinclift]

Awesome. 😄