Try SASL by default

[4e554c4c]

SASL PLAIN should be tried by default before USER/PASS authentication, so that authentication can be completed pre-registration. Halloy could detect whether SASL is allowed by doing capability negotation, or by opening an initial connection to test caps before the main connection (this approach is taken by goguma).

This would be nice so that instead of a config like

[servers.foo]
...
[servers.foo.sasl.plain]
username = "..."
password = "..."

one could just do

[servers.foo]
username = "..."
password = "..."

SASL is required for several things, including soju.im/bouncer-networks (#77). So if we want people to be able to easily use this feature, we should make SASL easier to use