refactor(pkg): refactor Plugin to make it easier to test

Writing the plugin's output string and exiting the program will no
longer take place in the plugin status itself. It will only be done in
the main program.

cmd/sslcert/main.go

@@ -215,9 +215,9 @@ func NewProgram() program.Program {
 	return program
 }
 
-// Terminate the monitoring check program.
-func (program *checkProgram) Done() {
-	program.plugin.Done()
+// Return the program's output value.
+func (program *checkProgram) Output() *plugin.Plugin {
+	return program.plugin
 }
 
 // Check the values that were specified from the command line. Returns true