Added g:taggatron_enabled configuration propperty

This allows a user to define common configuration parameters in a
single, global location and enable taggatron functionality on a per
buffer basis.
Prevented taggatron from clobbering an existing &tags value

Updated taggatron#SetTags() 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.

Values added to tagdefaults are appended to &tags upon plugin
initialisation. Thus, allowing users to define not just individual
tagfiles but also custom search patterns.

Updated taggatron#CreateTags() to make use of the taggatron#SetTags(),
instead of manipulating &tags value directly.
Replaced taggatron#makeAbsoluteFilePath with a buildin call to fnamemodify()

This one is compleatly arbitrary. There is absolutly nothing wrong with
makeAbsoluteFilePath(), but I feel that fnamemodify() would suit the
code better. Also, its use is consistent with taggatron#SetTags() code.
Improved code comments
Merge pull request #4 from UmkaDK/set-tags

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