macOS: use GNU instead of BSD grep by default
macOS: use GNU instead of BSD grep by default

BSD grep does not support digit pattern:
https://stackoverflow.com/questions/6901171/is-d-not-supported-by-greps-basic-expressions

$ grep --version
grep (BSD grep) 2.5.1-FreeBSD

$ ggrep --version
ggrep (GNU grep) 3.4

vogler/prezto