feat(tour) show list of topics when user tries to view topic that doesn't exist

eg.

```
ipfs2 (cmd-ref-part2) λ. go build ./...; ./ipfs2 tour 0

Tour 0 - Hello Mars

Hello Mars

ipfs2 (cmd-ref-part2) λ. go build ./...; ./ipfs2 tour 10

ERROR
    no topic with id: 10

TOPICS

    0 - Hello Mars
    0.1 - Hello Mars 2

```
feat(details) add funcs to negate negations

not immediately useful, but nice to have

tagging you to make sure i didn't make a mistake here @jbenet

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>