Formatter: Statistics

[philsturgeon]

User story.
As a spectral CLI user I'd like to see common offenders in a clearer way than line by line.

Describe the solution you'd like

A new formatter that can output something along these lines:

statistics
  Total number of errors   : 284
  Total number of warnings : 378
  errors
   1   (0%) : 'scopes' is required property type object. The available scopes for the OAuth2 security scheme.
   3   (1%) : Arrays MUST NOT be returned as the top-level structure in a response body.
   1   (0%) : Parameter names must follow case convention
  42  (15%) : Path segments must follow case convention
 219  (77%) : Property names must follow case convention
  18   (6%) : Enum values must follow case convention
  warnings
  66  (17%) : operationIds should follow naming convention
  93  (25%) : operationIds must follow case convention
   5   (1%) : Required parameters should appear before optional parameters.
  10   (3%) : Common path parameters should be defined on path object
   5   (1%) : Definition was declared but never used in document
 104  (28%) : Schema must have a non-empty description.
  61  (16%) : Schema properties must have a description with content in it.
  17   (4%) : Property has inconsistent type
  17   (4%) : Description sibling to $ref matches that of the referenced schema. This is redundant and should be removed.

Additional context

Suggestion from @mkistler.

[mnaumanali94]

Related to scoring discussion here: #2429