Add tls-min-version option

This adds a new tls-min-version flag that enables nsqd users
to specify a min tls version acceptable. The arguments to this
flag, <v>, follow the format of Version<v> in the tls package.

This commit parallels bitly/nsq#513 for issue bitly/nsq#512.
Merge pull request #98 from twmb/tls_min_version_512

go-nsq: add tls-min-version config option