Add "os tree" diagram

[Pedro-Montanari]

Hello,

first of all, kudos to the mermaid devs for their amazing work.

I would like to request the addition of an "os tree" diagram. Something like this, for example:

Mermaid input

.
`-- root/
    |-- foo/
    |   `-- bar
    `-- baz

Mermaid output

I think it would fit well with mermaid capabilities and use-cases. Thank you