Log Message:
Add xe, a simple xargs and apply replacement.

`xe` is a new tool for constructing command lines from file listings or
arguments, which includes the best features of `xargs(1)` and
`apply(1)`. `xe` means "execute for every ...".

Benefits over xargs:
* Sane defaults (behaves like `xargs -d'\n' -I{} -n1 -r`).
* No weird parsing, arguments are separated linewise or by NUL byte.
* Can also take arguments from command-line.
* No shell involved unless `-s` is used.
* `{}` replacing possible with multiple arguments.
* Support for patterns to run different commands depending on the argument.

Benefits over apply:
* Parallel mode.
* Sane argument splitting.
* Can use shell-syntax instead of escape characters.