and upgrade "husky" to v1.3

Any commands with a --yarn flag will look for a "yarn.lock" file in the working directory. When a lockfile exists, the --yarn flag will be true by default.

Other fixes:
- actually use argv.yarn for the "link" command
- actually use the `getPackageManager` function's argument
- remove unused `showVersion` function

Make the `add` and `link` commands update the "node_modules/.bin" directory.

Yalc now looks for the nearest ancestor that contains a package.json file.

I also did some tidying up!

Fixes wclr#47

Improve the "remove" command by cleaning up after ourselves in the "node_modules/.bin" directory.