support both square brackets and raw syntax for IPV6

[ndeloof]

as discovered on docker-archive/compose-cli#1386:

docker-compose (docker-py) uses raw IPv6 for port specification, which go-connections reject as it relies on net.SplitHostPort which require square bracets.

This PR relax requirement for square bracket, while still supporting those. I'll prepare another PR on docker-py to accepts and ignore square brackets, so both can be interoperable.

this is required to let IPV6 users (our own @chris-crone might not be the sole one) switch between docker-compose to dockercompose

[gtardif]

LGTM

[thaJeztah]

LGTM

[thaJeztah]

Argh.. #77 still isn't merged yet I see, so tests don't actually run 🙄

[thaJeztah]

Thanks! all tests now ran, and passed

=== RUN   TestPartPortSpecIPV6
=== RUN   TestPartPortSpecIPV6/square_angled_IPV6_without_host_port
=== RUN   TestPartPortSpecIPV6/square_angled_IPV6_with_host_port
=== RUN   TestPartPortSpecIPV6/IPV6_without_host_port
=== RUN   TestPartPortSpecIPV6/IPV6_with_host_port
=== RUN   TestPartPortSpecIPV6/::_IPV6,_without_host_port
--- PASS: TestPartPortSpecIPV6 (0.00s)
    --- PASS: TestPartPortSpecIPV6/square_angled_IPV6_without_host_port (0.00s)
    --- PASS: TestPartPortSpecIPV6/square_angled_IPV6_with_host_port (0.00s)
    --- PASS: TestPartPortSpecIPV6/IPV6_without_host_port (0.00s)
    --- PASS: TestPartPortSpecIPV6/IPV6_with_host_port (0.00s)
    --- PASS: TestPartPortSpecIPV6/::_IPV6,_without_host_port (0.00s)

[thaJeztah]

LGTM