Add steep stats command

[soutaro]

steep stats will show a statistics on method call typings.

# Example
$ bundle exec steep stats | column -s, -t
Target  File                            Typed calls  Untyped calls  All calls  Typed %
lib     lib/rbs/ast/annotation.rb       12           1              13         92.31
lib     lib/rbs/ast/comment.rb          14           1              15         93.33
lib     lib/rbs/ast/declarations.rb     289          44             337        85.76
lib     lib/rbs/ast/members.rb          206          24             230        89.57
lib     lib/rbs/buffer.rb               32           0              32         100.00
lib     lib/rbs/builtin_names.rb        43           0              43         100.00
lib     lib/rbs/cli.rb                  152          276            518        29.34
lib     lib/rbs/constant.rb             18           5              23         78.26
lib     lib/rbs/constant_table.rb       112          1              116        96.55
lib     lib/rbs/definition.rb           181          1              183        98.91