Make text areas configurable

Names of text areas which should have snippets available can now be configured via `plugin_Snippets_textarea_names` configuration option (default: "bugnote_text"), i.e. "bugnote_text, steps_to_reproduce". It makes including other fields easier than hard-coding each new field into code like proposed in [issue 3](https://github.com/mantisbt-plugins/snippets/issues/3)
I don't really like your indentation here, better 
- put the closure's declaration on line 41, on same indent as the closing `}`
- Snippet::get_configured_field_names() on a line of its own