Add --root global argument. Fixes #899.

[etherealvisage]

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Code changes

• Are you doing the PR on the next branch?

If the change is a new feature or adding to/changing an existing one:

• Have you created/updated the relevant documentation page(s)?

Notes

This does pretty much what it says on the tin :-) Fairly new to rust and this was a quick patch so apologies if I've not done things quite the way you'd like.

[Keats]

Looks good, thanks!