Improve get command arguments validation

[qzb]

Currently get command validates its parameters rather loosely. We should add at least following checks:

• check if number of positional arguments is less than 2
• validate command name and --alias against regexp (/^[a-z0-9_-]+$/?`)
• check if --version is a correct semver range
• check if --from is a correct URL

[crazybarber]

First bullet point is resolved by #41