shards remove command

[devnote-dev]

Following the naming of #144 and #144 (comment), have a shards remove command to remove a dependency/dependencies from shard.yml. Currently, if you want to remove one or more dependencies from your project, you have to do the following:

• remove the dependencies from shard.yml
• run shards update
• run shards prune

While this process isn't necessarily wrong, it's not intuitive either – new Crystal users aren't going to know that this is the process as it isn't really documented, nor does it imply that what you're doing is essentially what a remove command would do. Furthermore, if you mistakenly skipped running shards update then those dependencies are stuck sitting in the lib folder which can lead to unexpected hidden issues.