Support download of run-command (rc) file.

[rduplain]

Broadly, qwerty.sh aims to allow vendoring of external files without checking them in. The previous issue introduces how rc files open up more use cases for qwerty.sh:

The run-command (rc) feature of qwerty.sh opens it up as a meta package manager, a tool that can package anything on the internet, especially with rc-file chaining (i.e. rc files that reference rc files) and proliferation of rc files with conditional execution of --arch, --sys, and --when.

... and rc files themselves become candidates for vendoring without checking in.

Currently, qwerty.sh does not support additional arguments with --rc. This opens up the possibility of --rc - indicating that qwerty.sh should use other arguments -- those which indicate what to download -- as files to pass into --rc.

Note that the Unix convention of - indicating standard input (stdin) is not readily supported by qwerty.sh because in its default configuration, the qwerty.sh script itself is passed on stdin. This fact makes it possible for --rc - to mean "that which qwerty.sh downloads" as a more abstracted interpretation of the - Unix file convention.

[rduplain]

--cd-on-rc and -o, --output would not be supported in --rc - mode, because qwerty.sh would need to download to a temporary directory when running rc files.