Support reading one import from stdin

[mkroetzsch]

For some uses of Nemo, it would be nice to pass data through stdin. Currently, one would have to write data to a file to pass it to Nemo.

Nemo already supports the use of "-" as a resource in @export to write to stdout, and the same syntax could work for @import and stdin. In addition, Nemo should check that at most one stdin source is used.