Feature: Add a setting for unsorted listing

While many like the structure view to list the matches sorted by type and name, others prefer to see the matches unsorted in the order of appearance in the document.

For ctags, adding `-u` to the command arguments will do the trick.

Thanks for considering this feature.

(Btw, I've just submitted a PR #22 to add support for RSpec and MiniTest which makes much more sense if you can switch off sorting.)

Start by locating the ctags command construction and the structure-view sorting or configuration entry points. Add a setting that controls whether ctags receives -u and preserves document order when enabled. Verify both sorted and unsorted listings in the Atom package, including the RSpec and MiniTest use case mentioned in PR #22.

Written by the indexing model from the issue text.