Add support for the "size" response of the "ipfs add" command

[timothyparez]

The following was merged into IPFS 0.4.11-rc: ipfs/kubo#4082
Will have to check this out and add support for it.

[lvermeulen]

Please note that getting the size on disk requires some platform-specific code on Windows. Therefore it is not possible to compare the size returned from ipfs add.