Prevented taggatron#SetTags() from clobbering an existing &tags value

- Updated taggatron to take into account the existing value of &tags, ie: if the tag file is already picket up by the current &tags then tags are not updated. Otherwise, the path to the tag file is appended to the end of the current &tags value.

- Added g:taggatron_enabled which allows user to temporarily disable taggatron functionality.

- Improved code comments