feat/tagfunc: 'tagfunc' now takes two arguments.
The first argument passed to 'tagfunc' is the pattern to be searched.
The second argument is a string - a set of flags which may be used to
control the behaviour of 'tagfunc'. The only flag that may appear as the
second argument at this time is 'c', which indicates that 'tagfunc' may
want to look at the context around the cursor to improve the search
result. In other words, the tag search is being performed due to a
normal command being processed, not a command from the command line.