Add manual indexing function

Rupa,

I just found z today - it's awesome, thank you.

At first install I was eager to get most of my `~/` indexed, but didn't want to do all that `cd`'ing... So I ran a `find` command on my favorite directories and appended the results + frecency format  to `~/.z`. Thought that may be a cool function to add in.

Command is set to 'zi [#_of_subdirs]', runs from user's current workig directory. Would have preferred 'z -i [#_of_subdirs]' but couldn't get the argument/scope to work out with the existing setup.

If anything about the pull request is bad, please let me know! I'm newly into this open source business, any feedback is greatly appreciated.

Thanks again,
--Sean