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)