add support for mdfind

In Mac, locate is not available by default because it requires database.
Instead of locate, you know, mdfind is available in Mac.
This commit adds support for mdfind for Mac user who doesn't use locate.

In addition, I decided to use {query} and {count} placeholder because
printf's formats must follow the order. They aren't available for mdfind
because a query of search must be before max count of candidates in
a command.
Merge pull request #3 from rhysd/add/mdfind_support

add support for mdfind