[EXPERIMENTAL] Extend the API of `Coverage.start` and `result`

The old API:

    Coverage.start
    load "foo.rb"
    p Coverage.result #=> {"foo.rb" => [1, 2, nil]}

The new API:

    ENV["COVERAGE_EXPERIMENTAL_MODE"] = "true"
    Coverage.start(lines: true)
    load "foo.rb"
    p Coverage.result #=> {"foo.rb" => {:lines => [1, 2, nil]}}

This new API allows us to add other measuring target types.

Notes:

* To keep compatibility, the old interface is still remained; passing no
  optional argument makes `Coverage.result` return the old format.
* This feature is still experimental.  So, to enable the new API, the
  environment variable `COVERAGE_EXPERIMENTAL_MODE` must be set.  I plan
  to activate this feature by Ruby 2.5.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59874 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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