RFR 8013252: Regex Matcher .start and .end should be accessible by group name

[Xueming Shen]

Hi,

The regex named capturing group support was added into jdk7 [1].
Matcher.group(gname) is the only direct access method we added back then
to access the matched result. The proposed change here is to add a pair of
accessing/convenient method Matcher.start/end(gname) to access the start/end
offset info of the matched result, to match the corresponding start/end/group
(int index) access methods.

http://cr.openjdk.java.net/~sherman/8013252/webrev

Thanks!
-Sherman

[1] http://cr.openjdk.java.net/~sherman/6350801/webrev.02/