Refactoring sub-commands as standalone commands under 'cmd/cobe-*'.

- This decreases massing of dependencies in the main command tool and
  allows to extract commands into their own projects should they grow
  too large easily.

- Starting to use cli pkg; adding usage and help information to flags.
  This harmonizes and increases display of usage information.

- Consequently using '-b, --brain' flag to allow specifying brain files.

- Dropping CPU profiling support from tool. Creating benchmark test
  with profiling enabled instead. Adding init subcommand for tool.
Implementing minimal REST HTTP server.