RFR: Reports plugin

[Alexandre Iline]

An initial code for the report plugin.

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Commit messages:
 - README
 - make context lighter
 - The report shoudl really be showing _source_ coverage, not class coverage.
 - remove pres
 - Allow anything in header.
 - Results from the same line ovverride previous results
 - add header and title to the report
 - Initial commit of a report building plugin.

Changes: https://git.openjdk.org/jcov/pull/41/files
 Webrev: https://webrevs.openjdk.org/?repo=jcov&pr=41&range=00
  Stats: 12373 lines in 35 files changed: 12371 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jcov/pull/41.diff
  Fetch: git fetch https://git.openjdk.org/jcov.git pull/41/head:pull/41

PR: https://git.openjdk.org/jcov/pull/41