Add g:FerretGrepCommand

[kriansa]

This change adds a new command to customize the grep command executed by
Ferret under the hook. For more context, see:

#38

[wincent]

Looks pretty solid. Thanks! I'll give this a proper review when I am at a computer, but this looks pretty good (it even has docs!).

[kriansa]

Hello @wincent!
Sorry to bother you, but is there something I could do to help to get this merged? I know you're probably busy, but let me know if there's something in this PR that you didn't feel comfortable with so I can fix it.
Thanks!

[kriansa]

Hi @wincent, did you find time to review this PR?

[kriansa]

Hi @wincent,

Sorry to bother, it's just that this is going on for almost a year now and you didn't review it. I would like to know if this is still something that you care about (fixing #39). It's fine if you don't have interest, or if you find that this solution sucks, but I would like to get an answer. I make myself available if you think that this is not the right way to do it, or if you would like me to add more information. I just want to help.

Thanks for the awesome work on this project! 😄

[wincent]

Thanks for you work on this @kriansa. In order to address a broader set of use cases (see #55) I've merged in something that allows you to set the arguments for each executable as opposed to supplying a string that overrides the executable + the args. Hopefully this will meet the need you're addressing in this PR and also the one in the other PR.