Feature: add array support to config.angular_templates.ignore_prefix

This change will make it possible for the `config.angular_templates.ignore_prefix` option to take an array of values. Each value will be ignored if it is at the start of the templateUrl string.

For example: `config.angular_templates.ignore_prefix = %w( templates/ legacy/templates/ )` means your templateUrl paths can be shortened if you have more than one template folder:
`'templates/nav_bar.html'` => `'nav_bar.html'`
`'legacy/templates/old_alert_window.html'` => `'old_alert_window.html'`

`config.angular_templates.ignore_prefix` still supports a string option for backwards compatibility.
This is the longest line in the file, and it uses an awkward syntax. It would be much better if you used if after the statement instead of before:
```ruby
# cleaner one line
configuration.ignore_prefix = [configuration.ignore_prefix]  if configuration.ignore_prefix.is_a? String

# or break it into multiple lines

if configuration.ignore_prefix.is_a? String
  configuration.ignore_prefix = [configuration.ignore_prefix]
end
```

**However** this code doesn't belong here at all. This statement should be executed once in the engine, not every time a template is rendered.  [In engine.eb after the `before_initialize` block,](https://github.com/AaronV/angular-rails-templates/blob/feature/ignore_prefix_array_support/lib/angular-rails-templates/engine.rb#L61)

```ruby
config.after_initialize do |app|
  app.configuration.ignore_prefix # more logic here
end
```