Add support for `(?<name>expr)`.

[junyer]

This follows https://github.com/google/re2/commit/6148386 (and https://github.com/golang/go/commit/ee61186) to some extent.

[codecov-commenter]

Codecov Report

Merging #167 (d2152b7) into master (e3c736d) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #167   +/-   ##
=======================================
  Coverage   89.06%   89.07%           
=======================================
  Files          19       19           
  Lines        3037     3038    +1     
  Branches      618      619    +1     
=======================================
+ Hits         2705     2706    +1     
  Misses        189      189           
  Partials      143      143           

Files Changed	Coverage Δ
java/com/google/re2j/Parser.java	88.02% <100.00%> (+0.01%)	:arrow_up: