Allow to import WXR file through HTTP and STDIN

[miya0001]

It will be useful if we can import WXR file through HTTP.
Following is an example to import theme unit test data of the theme review team.

$ wp import https://raw.githubusercontent.com/WPTRT/theme-unit-test/master/themeunittestdata.wordpress.xml --authors=import

Also, I think it should to be able to import from STDIN like following and it looks useful.

$ wp export --stdout | wp @staging import - --authors=import

[nreynis]

Importing from stdin is a must have for containerized instances

[nhh]

I am also for importing via STDIN which makes it super convenient to work with kubectl, s3cmd's etc.