Remove warning about non-existent instance var (#42)
Remove warning about non-existent instance var (#42)

In the current code the first time a template is rendered you will get this warning:

```
./Users/rschneeman/Documents/projects/maildown/lib/maildown/ext/action_mailer.rb:48: warning: instance variable @maildown_text_template not initialized
```

This is because `@maildown_text_template` is not yet initialized. Instead, we can check to see if it exists first, before trying to use it using `instance_variable_defined?`.
schneems/maildown,schneems/maildown,schneems/maildown