Overriding numbered members is not intuitive.
Merging {1} and {2} should return {1,2} not {2}.

Change vis_sel_to_pos to vis internals or to count lines itself

Closes #38

See merge request muhq/vis-lspc!58

Check a vis:pipe function call like we would call it in vis_pipe.
Omitting the fullscreen parameter causes the pcall to succeed, although vis does not support
using a string as data source.

Use the the internal vis logic as fast path if available.

- Use `util.vis_pipe` instead of `vis:pipe` as this does not work on the
  current release of vis.
- Quote the path to the find-upwards script. It resides in
  `~/Library/Application Support` on my macOS setup, and while not very
  common, paths in other users' configurations might have the same
  issue.
- At least during testing, `out` was nil in some cases. `find_upwards`
  checks it now.