Allow array of paths for partials

[jimlambie]

This PR allows specifying an array of paths for partials

In config:

"paths": {
    "partials": [
        "workspace/partials",
        "../../frontend-system"
    ]
}

This gives the ability to reference partials from Dust templates using the format last-part-of-directory/partial-name

For example, given a template called gallery and using the paths above:

{>"partials/gallery"/}
{>"frontend-system/gallery"/}

Close #108
Close #115
Close #117

[abovedave]

Tested and confirmed working for #117 (including when symlinking the foreign folder). Many thanks @jimlambie

[eduardoboucas]

Looks good to me!