Refactoring of pb_list() and other changes

[tanho63]

• pb_releases() created and exported, returns a list of releases available in repo
• pb_info rewritten to handle multiple and specific tags, resolves pb_list() will soon fail if multiple tags are specified #61
• pb_info now checks for specified tags first and only returns assets for those tags if specified
• pb_info no longer asks about creating new releases, nor does it warn about missing releases - resolves pb_list() should not offer to create a new release if no releases are found #60
• pb_upload asks about creating a new release if no release found - correct spot to ask about releases in pb_list() should not offer to create a new release if no releases are found #60
• memoise moved to .onLoad() per best practice, resolves memoise should happen onLoad and not at build time #63
• updating CLI messaging to use {cli} instead of {clisymbols} and {crayon} to reduce dependencies (gh imports cli and that seems to be latest best practice) resolves simplify various CLI messaging package dependencies #64

TODO:

• make all tests pass
• update news file
• audit rest of code for print messaging improvements

[tanho63]

Okay, so this PR got a little bit away from me and does quite a lot, but I think it takes care of a lot of the outstanding issues I noticed when I was working through it.

Please let me know if you see any issues or have any questions!

[cboettig]

All looks good to me. Thanks for the excellent work here! Will cut a new CRAN release to ship these changes shortly.

[tanho63]

Awesome! Let me know if there's anything I can help with re: CRAN release