Check `args` slice bounds in all commands

[waits]

Several commands panic when not passed enough arguments. They print an unhelpful index out of range error and a stack trace. This is undesirable for most routine error conditions.

In the same spirit as #12 I added len(args) guard clauses to the remaining commands that require arguments. They now return a helpful error explaining the arguments needed.

Example:

$ dbxcli mkdir
Error: `mkdir` requires a `directory` argument

Thoughts/questions:

• Should this be DRY'd into a validateArgs function in root.go? It's only three lines per command, so I wasn't sure if it's worth it.
• Should we print a longer error message including the usage text? We already have that text available in the cobra.Command so it would reduce duplication. Many other CLI tools do this. Example from Docker:

$ docker pull
docker: "pull" requires 1 argument.
See 'docker pull --help'.

Usage:  docker pull [OPTIONS] NAME[:TAG|@DIGEST]

Pull an image or a repository from a registry

[diwakergupta]

Merging as-is, thanks @waits! Regarding your comments, I really wish Cobra had some way to support this directly (e.g. some notion of a "validator" for each command that can check on len(args) or other validations).