use gawk to extract token when egrep has no -o option

Some version of egrep has no -o (--only-matching) option, so use gawk instead.

I found the issue when using JSON.sh in MSYS and Git Bash console (installed via msys-git) on Windows. The version of egrep on that platform is 2.4.2 which seems too old. I think many people like me do not like to pay extra effort to search and download the newer version of egrep to get the script to work. I just want to simply drag the JSON.sh scrip to my folder and start using it.