Functionality: implement 'genhtml --select-script ..' callback to

select subset of coverpoints to show in HTML report.
The expected use case is code review, when we want to show only
the changes caused by a particular commit or set of commits - say, to
show the new code as well as any coverage gains or losses outside of
the code change.  See the '--select-script' section of the genhtml man
page, as well as the 'review' section in the 'example' directory.

Also modify report generation to indicate regions where baseline code
has been deleted.  These are show with grey background entries in the
'line number' column of the source detail window.  If the mouse hovers
over the grey area, a tooltip will appear to summarize the extent
of the deletion.

Signed-off-by:  Henry Cox <[email protected]>

README

@@ -238,8 +238,8 @@ New features and capabilities fall into 7 major categories:
      and the genhtml man page for details.
 
      Related options:
-        --baseline-file, --diff-file, --annotate-script, --date-bins,
-        --new-file-as-baseline, --elide-path-mismtach
+        --baseline-file, --diff-file, --annotate-script, --select-script
+        --date-bins, --new-file-as-baseline, --elide-path-mismatch
 
   b) Error handling
 
@@ -323,6 +323,9 @@ New features and capabilities fall into 7 major categories:
              Sample script: criteria
         iv) find source files in more complicated environments - where
             simple substitutions become complicated or unweildy.
+        v) select a subset of coverage data to display - e.g., to
+           use in a code review which wants to concentrate on only
+           the changes caused by a particular commit or range of commits.
 
      The callback may be any desired script or executable - but there
      may be performance advantages if it is written as a Perl module.
@@ -335,7 +338,7 @@ New features and capabilities fall into 7 major categories:
 
      Related options:
        --annotate-script, --criteria-script, --version-script
-       --resolve-script
+       --resolve-script --select-script
 
   f) Performance