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
this is so that we don't call os.Stat twice for every file matched
by Match(). Also changing the behavior to _not_ return the name of a
file that doesn't exist if it's not a glob.
Cameron Sparr