Add option to set cwd in `Liftoff` constructor

[weswigham]

This way the appropriate cwd can be set when all register hooks get hooked in. This is an issue in, for example, gulp when using ts-node in a subdirectory with --cwd. The cwd passed is not respected by the require hooks included by liftoff since the cwd is only set during launch (so it is not set when the require hook is loaded).

[phated]

The require hooks are loaded with node's require which doesn't take into account a cwd parameter and instead use process.cwd(). Liftoff should not be responsible for changing that (although, gulp decides to do that in its CLI). @tkellen do you agree?

[weswigham]

So the correct fix would instead be, in gulp, before it's construction of Liftoff, using process.chdir?

[phated]

@weswigham that would be the only place where we'd consider it. If you open an issue on gulp-cli, we can discuss over there. It would effect this chunk of code: https://github.com/gulpjs/gulp-cli/blob/master/index.js#L131-L137

Closing this.

[tkellen]

Agreed @phated! If we wind up thinking this is a best practice, we should consider updating hacker to do it as well.