Print usage on slinguist --help

[bzz]

It would be nice to have proper
In terminal

Actual usage as of v1.2.0

go get go get github.com/src-d/simple-linguist/cli/slinguist
slinguist --help
Usage of slinguist:

Expected: to have some explanation about flags, usage patterns --breakdown, json, etc.

Here is example of original github/linguist

linguist --help                                                                                       

  Linguist v5.0.8
  Detect language type for a file, or, given a directory, determine language breakdown.

  Usage: linguist <path>
         linguist <path> [--breakdown] [--json]
         linguist [--breakdown] [--json]