allow overriding lib/*.zsh in custom/lib
Merge pull request #3344 from wrboyce/custom-lib

allow overriding lib/*.zsh in custom/lib
Do not use GREP_OPTIONS to set grep parameters

From grep version 2.21, using environment variable `GREP_OPTIONS' to
set common parameters to grep calls is deprecated.

As an alternative, we use a custom function to pass in our defined
`GREP_OPTIONS`, while at the same time preserving any aliases that
called grep.

If we choose the alias way as suggested in the grep documentation,
we'll overwrite any already defined grep aliases, or get overwritten
by any subsequently defined user aliases.