* Adding iniitial draft for the addition of the count column on the histogram command
* Update histogram documentation
* Add count column test to histogram command
* Fix error in histogram documentation
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).
