refactor: "yalc add" flags

[aleclarson]

This changes the default behavior of yalc add to link to .yalc/{name} from node_modules instead of copying the contents of .yalc/{name}. You can use --no-link to get the old behavior.

This is better because it means any breakpoints I set in .yalc/{name} will be used no matter what; whereas the current behavior ignores those breakpoints if I use yalc add instead of yalc link.

And once #66 is merged, this commit lets us avoid copying .yalc/{name}/node_modules to node_modules/{name}/node_modules.

The relevant commit: d7a4f56fe7c24cd0eaddd79257feb5dd0e76d902

(Probably wanta merge #59 first)

[aleclarson]

Note to self: Cherry-pick https://github.com/aleclarson/yalc/commit/75c56cf5fc1d338e2134cd7acc07236adf105984 before this is merged.