Allow piping code from stdin

[ghost]

It'd be nice if regenerator assumed stdin as a file source by default, as convention generally suggests, instead of printing out help information when no arguments are given. While one can simply run regenerator /dev/stdin on Linux, not everyone knows about this file.

Example: echo 'var x = function*() { yield 1; yield 2; yield 3; }' | regenerator should print the generated code.