Document positional args in help text

[zzak]

Using a combination of the default UsageTemplate from cobra, and a custom function which we've added to cobra template, we can create our own template which will display args in the help text for commands that have annotations (which are documented here as a map[string]string).

This PR was created on top of #40 in hopes that will be merged first, and we can preview it's changes on this branch. Once that PR is merged, we can rebase this one and it should merge cleanly into master.

Demo

Here's the help text for a command without args:

$ go run main.go help namespace

Operate on namespaces


Usage:
  circleci namespace [command]

Available Commands:
  create      create a namespace

Flags:
  -h, --help   help for namespace

Here's the help text for a command with args:

$ go run main.go help namespace create
create a namespace


Usage:
  circleci namespace create NAME VCS-TYPE ORG-NAME [flags]

Args:
  ORG-NAME    The name used for your organization
  VCS-TYPE    Your VCS provider, can be either "github" or "bitbucket"

Flags:
  -h, --help   help for create

[codecov-io]

Codecov Report

Merging #43 into master will increase coverage by 1.29%.
The diff coverage is 41.58%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
+ Coverage   39.17%   40.47%   +1.29%     
==========================================
  Files          13       13              
  Lines         878      877       -1     
==========================================
+ Hits          344      355      +11     
+ Misses        510      498      -12     
  Partials       24       24

Impacted Files	Coverage Δ
cmd/build.go	38.94% <ø> (+4.61%)	⬆️
cmd/namespace.go	62.16% <100%> (+4.58%)	⬆️
cmd/update.go	26.31% <17.64%> (-4.84%)	⬇️
cmd/query.go	25.92% <27.27%> (-4.08%)	⬇️
cmd/config.go	43.03% <33.33%> (+1.78%)	⬆️
cmd/orb.go	27.11% <62.5%> (+3.06%)	⬆️
cmd/root.go	73.68% <66.66%> (-0.83%)	⬇️

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