Load external .so plugins

support for the Go 1.8 shared object feature of loading external
plugins.

this support relies on the developer defining a `Plugin` symbol in their
.so file that is a telegraf plugin interface.

So instead of the plugin developer "Adding" their own plugin to the
telegraf registry, telegraf loads the .so, looks up the Plugin symbol,
and then adds it if it finds it.

The name of the plugin is determined by telegraf, and is namespaced
based on the filename and path.

see #1717

internal/config/config.go

@@ -39,6 +39,14 @@ var (
 
 	// envVarRe is a regex to find environment variables in the config file
 	envVarRe = regexp.MustCompile(`\$\w+`)
+
+	// addQuoteRe is a regex for finding and adding quotes around / characters
+	// when they are used for distinguishing external plugins.
+	// ie, a ReplaceAll() with this pattern will be used to turn this:
+	//     [[inputs.external/test/example]]
+	//  to
+	//     [[inputs."external/test/example"]]
+	addQuoteRe = regexp.MustCompile(`(\[?\[?inputs|outputs|processors|aggregators)\.(external\/[^.\]]+)`)
 )
 
 // Config specifies the URL/user/password for the database that telegraf
@@ -704,6 +712,9 @@ func parseFile(fpath string) (*ast.Table, error) {
 		}
 	}
 
+	// add quotes around external plugin paths.
+	contents = addQuoteRe.ReplaceAll(contents, []byte(`$1."$2"`))
+
 	return toml.Parse(contents)
 }