spider: remove multi_site, visit takes 1 parameter

rename starting_host to host_limit
spider transition to urls not strings

- visit_page(URI)
- outstanding[URI]
- visited(URI)
rm visit_relative, local_href => want_to_visit
separate out limiter interface
better cover cli for mutants

cli is currently only outstanding tests for mutants
most (if not all) of these are related to the help text
unneeded rspec "once" since it is implied