Standalone

This commit adds a `-g` flag to generate a standalone shell script. This shell script can then be run again, even on different computers, to recreate the symlinks.

- This allows people to recreate the "download my dotfiles and run `./install.sh`" instructions, but with generated code.
- This provides us more freedom with lsrc(1): since rcm can be used to generate a universal shell script, lsrc(1) now can be harder to install -- it can depend on a compiler, for example -- because you only need to install it on one machine.
Should `$generate` be quoted?