Implementing cd('-') to behave like Bash's "cd -"

[nfischer]

I added a previousDir field to common.state to allow the cd() function to have this option. I also added unit tests for this behavior.

Usage is as follows:

// Setup
mkdir('foo');
mkdir('bar');
cd('foo');
cd('../bar');
echo(pwd()); // outputs "/path/to/bar"
cd('-'); // jump back
echo(pwd()); // outputs "/path/to/foo"
cd('-'); // jump back again
echo(pwd()); // outputs "/path/to/bar"

[nfischer]

This can already be done by something like:

var prevDir = pwd();
cd('foo');
// do something
cd(prevDir); // jump back

This PR just makes things a little easier if you're porting sh to shelljs, and makes things a little more convenient for that use-case. This isn't a critical feature, but it doesn't cause too much harm, so I figured I'd implement it.

[ariporad]

Maybe make this error message a little more descriptive? OLDPWD takes a minute to understand.

[nfischer]

Sure thing. I just copied the error message from bash. I agree, it probably should be more descriptive.

[ariporad]

@nfischer: Just a few small things, otherwise great job!

[nfischer]

@ariporad this should be good now

[ariporad]

LGTM!