Add driver for shell command

[ejboy]

Some ideas:

Config

Allow to configure which shell executable to use, e.g.

shell.posix.cmd_name = /usr/bin/sh
shell.win.cmd_name = cmd.exe /c 

Dialects

It should be possible to specify os family in the dialect, to write cross platform scripts
os.name/os.version properties will work.

Queries

<query>
   for i in {0..3}
do
  echo "Number: $i"
done
</query>

Should produce lines as the result of the query.