RFR: 80: Allow valid bug ID format to be configurable

[Kevin Rushforth]

JBS issue: https://bugs.openjdk.java.net/browse/SKARA-80

This provides a configuration option to define the pattern for a valid issue ID as follows. This starts with the idea @edvbld mentioned in PR #99, although I felt it was cleaner to leave the default issue format as is, and use the more restrictive pattern for the "legacy" jdk conf file.

The syntax of the new pattern is:

[checks "issues"]
pattern=<PATTERN_STRING>

The default pattern is:

[checks "issues"]
pattern=^(([A-Z][A-Z0-9]+-)?[0-9]+): (\\S.*)$

For legacy JDK `.jcheck/conf` files, the pattern is as follows to match the issue ID rules in `jcheck.py`:

[checks "issues"]
pattern=^([124-8][0-9]{6}): (\\S.*)$

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Commits:
 - a12ec8ee: 80: Allow valid bug ID format to be configurable

Changes: https://git.openjdk.java.net/skara/pull/163/files
 Webrev: https://webrevs.openjdk.java.net/skara/163/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-80
  Stats: 321 lines in 5 files changed: 315 ins; 1 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/163.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/163/head:pull/163

PR: https://git.openjdk.java.net/skara/pull/163