Add a builder for ValidatorChains.

This allows them to be created inline without using type-unsafe
varargs (albeit a bit more verbosely).

new ValidatorChain(v1, v2, ...) becomes

ValidatorChain.builder()
    .addValidator(v1)
    .addValidator(v2)
    ...
    .build();

Change-Id: I6c92142bf294335419fbd16593125416dd33c74e
Reviewed-on: http://gerrit.dmdirc.com/3168
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>