Shell command execution

Implemented the following features:

1. External executable search (some features still are Windows-only): by absolute path, by relative path, in placed pointed by system `PATH` variable.
2. Basic external process interoperability. Execution has two modes: when in middle of pipeline (for example, `$x = ping`), process output will be catched and send to pipeline. Else, process will work interactively (useful with any REPL, for example with `python`).

Next step will be Unix support and any feature requests.

Also, I found some glitches with parsing (that is the case for some tests disabled). I'll investigate them and document the issues.