[Feature] Enchance text_part:get_content method

[Feature] Enchance text_part:get_content method

This method now supports otional `type` attribute which could be
following:

- `content` (default): utf8 content with HTML tags stripped and newlines preserved
- `content_oneline`: utf8 content with HTML tags and newlines stripped
- `raw`: raw content, not mime decoded nor utf8 converted
- `raw_parsed`: raw content, mime decoded, not utf8 converted
- `raw_utf`: raw content, mime decoded, utf8 converted (but with HTML tags and newlines)