RFR: Initial commit of data plugin

[Alexandre Iline]

Initial commit of data coverage JCov plugin. See the readme and tests.

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Commit messages:
 - Initial commit of data plugin

Changes: https://git.openjdk.java.net/jcov/pull/16/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcov&pr=16&range=00
  Stats: 1249 lines in 26 files changed: 1249 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jcov/pull/16.diff
  Fetch: git fetch https://git.openjdk.java.net/jcov pull/16/head:pull/16

PR: https://git.openjdk.java.net/jcov/pull/16