this is for supporting external plugins.
external plugins will depend on a few telegraf interface types, as well
as a common telegraf registry.
this will allow external and internal plugins to both share this package
and make it easier to vendor/version the whole thing semantically, which
will make it easier to keep plugins supported across build and telegraf
versions.
see #1717
Also changing the net_response and http_response plugins to only accept
duration strings for their timeout parameters. This is a breaking config
file change.
closes#1214
Cameron Sparr
Thibault Cohen
Thibault Cohen