New bullet styles for TODOs and questions

[jameslairdsmith]

Fixes #779.

Also touches on the subject matter of the usethis vignette "Converting usethis's UI to use cli" because of the TODOs.

Proposed new syntax:

cli_bullets(c("?" = "This is a question", "[]" = "This is a TODO"))
#> ❓ This is a question
#> ☐ This is a TODO

Some outstanding questions:

• Do we want to use the fancy question mark instead of the regular one?
• Is red the correct colour? It felt correct to me for the TODO given that usethis::ui_todo() used red as well.
• The usethis vignette proposes the underscore "_" as the LHS for TODO instead of "[]". The "[]" feels more natural to me, but it is two characters whereas all the others are one.

Other things.

• Using roxygen2 (even the development version) seems to be broken for me because of a problem unrelated to the PR. It's in the "cliapp-docs.R".