Fix missing attachment field in options page, implement max email clip size, misc bug fixes

Added support for email attachments.
Various cleanup in source.

- Major refactor to centralize functionality into background.js to support future features.
- Added ability to clip selected text. (Issues 12 & 22 at github)
- Added ObsidianClipper to menu. (Issue 18 at github)
- Added status line messages.
- Various cleanup and bug fixes.

Added CC/BCC features and YAML support for same.

Changed colored tags to use CSS instead of broken HTML.

Added theses new features:
- Clickable email IDs - Add '[click here to open](_MSGURI)' to a template to click in Obsidian and open original email in Thunderbird.
- Message tags - Add '_MSGTAGSLIST' in your template to get a list of Thunderbird tags in color.
- Additional time/date fields - Use Year/month/date/hour/minute/second to format time any way you want.
- New character substitution options
- General cleanup and refactoring.

Improved ability of user to exclude or substitute characters in filenames.
Improved documentation.