Modularize internal commands

[woodruffw]

Right now, the kbsecret command calls internal commands (like list) via exec, just like external commands:

elsif external?(command) || internal?(command)
  exec expand(command), *args

This is slow, since it requires Ruby (and all dependent libraries) to be loaded twice: once in kbsecret itself, and once in the exec'd subcommand. It also makes it harder to generate code coverage on the CLI.

In order to avoid this reloading, all internal kbsecret commands should become first class objects under the KBSecret::CLI namespace. Some options:

• Subclass KBSecret::CLI in each, like KBsecret::CLI::New to kbsecret new.
• Create a new subhierarchy with a consistent mapping, like KBSecret::CLI::Command::New to kbsecret new.

[woodruffw]

Based on local testing, this results in a decent performance boost: require "kbsecret" takes about 400ms in total, so preventing a subcommand from needing its own brings a ~30% speedup in many cases.

And a solid half of require "kbsecret" is just making sure KBFS is mounted:
4920bc9 addresses this:

[naveenanilabs]

Anyone working on this i want to help you guys.

[woodruffw]

Nope, nobody is. Feel free to take a stab!

[woodruffw]

Began work on this as #67.