Added g:taggatron_enabled & prevented taggatron from clobbering an existing &tags value

Added g:taggatron_enabled allow users to define common configuration parameters in a single, global location and enable taggatron functionality on a per buffer basis.

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.

See commits comments for a fuller description. :)