Implement && and || Operators

[mmstick]

Resolves issue #227 by implementing support for && (AND) and || (OR) operators. There's also basic background support, so background jobs will run in the background and print when they are complete.

The following command:

echo one && echo two && echo three

Will print

one
two
three

While the following:

echo one && echo two || echo one

Will print

one
two

And the following:

ecsdf one && echo two && echo three || echo four

Will print

ion: command not found: ecsdf
four

[mmstick]

Update: And it's fixed now.

[jackpot51]

Thanks @mmstick! Any ideas on how to fuzz the grammer parser?

[mmstick]

Not that I know of, besides continuing to add more and more unit tests, and adding a more comprehensive suite of scripts to test against.