Add an exec command

[thosakwe]

[dumblob]

Do you mean exec of external binaries/apps or rather an "internal" exec (thus something like eval or better quoting/unquoting as proposed in https://github.com/thosakwe/t2b/issues/17 )?

If you meant internal one, I'd prefer the quoting/unquoting approach. But that leaves one important question open - namely how t2b can read from stdin? That's probably the last thing (along those proposed in #17) I lack in t2b - reading from stdin.