Xontrib-pipeliner

**plx**
* Print command to stderr instead of stdout.

* Fix `ppl` command for Mac
* Added "Known issues" for `ppl` command on Mac

Compiled python.

PyPi package maintenance.

New experimental feature - syntax highlighting using xonsh prompt:

If you're using xonsh prompt and want to use pipeliner with syntax highlighting instead of string there is experimental
feature that catch `pl (<python>)` calls and uses the expression from the xonsh python substitution as pipeliner argument.
Example:

bash
echo echo | pl (line + '!')
In the xonsh prompt it's equals to:
echo echo | pl "line + '!'"

Now you can begin the pipe from Python head:
bash
pl "'\\n'.join(list('ABCDEFG'))" | pl "line + ('!' if num%2 else '?')" | grep '!'


B!
D!
F!