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)
Probably better not to mention plugin_Snippets_textarea_names at all, so that all config takes place in the config page and we don't incite people to hack the backend.