Don't Join/Split command template

Currently the command template is a single string that is Joined from flag.Args() and later split again on space. This destroys argument boundaries if any of the arguments or the placeholder replacement contains a space.

For example:

    $ echo "foo" >> "has space.txt"
    $ fzz grep {{}} *.txt
    >> f
    grep: has: No such file or directory
    grep: space.txt: No such file or directory

    $ fzz grep {{}} *.go
    >> replacement has space
    grep: has: No such file or directory
    grep: space: No such file or directory

Instead of Join/Split, just keep the template in a []string preserving the argument boundaries.