Add support for nREPL sideloading

[celiocidral]

Sideloading is useful when e.g. you want to run an nREPL in some remote host and you want to load source files from your local host. The assumption is that the remote host doesn't have any source files.

For more information on sideloading: https://nrepl.org/nrepl/0.9/design/middleware.html#sideloading