multiext support for named output

[fgvieira]

Is your feature request related to a problem? Please describe.

Right now it is not possible to cache a rule's output when there are several named output files. With several output files we can use multiext, but it does not allow for named output.

Describe the solution you'd like

I was thinking that it would be nice to have a way to have multiple named output files with multiext, for example:

output:
    multiext("path/to/output_file.", out1 = "ext1",  out2 = "ext2")

or:

output:
    out1, out2 = multiext("path/to/output_file.", "ext1", "ext2")

[notestaff]

I'd like to second the request for this feature.