Add path resolver to `path_or_url`

[joverlee521]

Prompted by nextstrain/avian-flu#156 (comment)

this does make the inputs/additional_inputs params different from other config params that expect file paths: they must always be relative to the working directory.

That's a subtlety of path_or_url, not inputs/additional_inputs per-se. We could add a path-resolver argument to the function and/or an explicit check that the requested local file path exists. (The reason we don't do this is that we can use the function to create / write files, and perhaps some other complexity that's slipping my mind currently.) Alternatively, this goal could be achieved by supplying search-paths to the consuming augur command.