streak-mode: require 'conceal' feature
move highlight group declarations to autoload/hl.vim

this nicely avoids the 'autocmd ColorScheme' workaround, because
it guarantees that our declarations occur after the colorscheme.
defer search-object initialization until autoload
vertical-scope-mode precludes streak-mode
doc
always remove normal sneak-highlight after exiting streak-mode
streak-mode: show cursor position while waiting for input

cursor may already be at the desired location, so this helps the user
know that <Esc> is the best choice
streak-mode: mask un-labeled matches and "tail" of labeled matches

restore user's original Conceal highlight-link, if any
streak-mode: treat case the same as normal sneak.
test: sneak#hl#links_to()
fix #26: streak-mode target label is wrong if line has a tab/linebreak/etc

- searchpos() returns byte-column, so it makes no sense to then
  tell :syntax that it is a virtual column.