feat(prereleases): add --pre option to install, search, add

[zeroshade]

closes #289

To test with the staging registry, you need to use the --no-verify option as the current pre-release in the staging directory is signed with the nightly key, not the standard private key.

[ianmcook]

Looks like there's a bug in the Available Versions output in dbc search --pre -v.

[ianmcook]

For the search subcommand, the --help description for --pre looks good.

But for the install and add subcommands, can you please change it to:

Allow implicit installation of pre-release versions

[ianmcook]

When I explicitly specify a version constraint like dbc install foo=v0.1.0-alpha.1, dbc fails with this error when it should succeed:

Error: error getting driver: no package found for driver foo that satisfies constraints =v0.1.0-alpha.1

[ianmcook]

Update: the two apparent bugs that I described in the comments above were caused by an erroneous platform tuple on the registry index that I was testing with, not by errors in this PR.