Files: Support multiple paths in the mkdir operation

[nathandf]

Issue:
Currently, the Files API mkdir operation only supports creating a single path per call.

This has two negative consequences.

• The first is, if the mkdir is not a true analog of the bash mkdir -p as the Linux operation Not that big of a deal, but that's what we tell users)
• The second is, because we do not support the exact feature offered by mkdir, we have to make multiple API calls if we wanted to do something equivalent to mkdir -p path/1 another/1

Potential Solutions:

• Allowing the path property in the request body to be either a string or an array of strings
• Adding a mkdirs operation - This is probably the easiest solution