Making git grep ignore binary the default

I've found that there is a flag (`git grep -I`) to ignore binary files, it works great, but I've found no way to make it the default.

It would be great to have a config for this. This way a possible implementation implies:

- Adding a config `grep.ignoreBinary` that defaults to false, keeping
the current default.

- Adding a flag `git grep --include-binary` to revert the default. But
maybe the `-a, --text` flag already does that.

Also, maybe the next git version (3.0) can default to ignore-binary as a better default.

Finally, if this makes sense, I can do my best to implement the change in the code.

Regards.
- Eloy