Move plugin registration to separate package

Only compile in plugin support on non-windows 1.8.
Re-enable go 1.7.x tests.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

plugin/plugin_other.go

@@ -0,0 +1,8 @@
+// +build !go1.8 windows
+
+package plugin
+
+func loadPlugins(path string) error {
+	// plugins not supported until 1.8
+	return nil
+}