make headpath configurable

If you have liveswap globally installed on a mac/linux and you run it without `sudo`, you'll get this error (or similar):

```
≫ liveswap -s instance/mmp-test-net.js --pre-upgrade ./lib/pull.js 

fs.js:427
  return binding.open(pathModule._makeLong(path), stringToFlags(flags), mode);
                 ^
Error: EACCES, permission denied '/usr/local/lib/node_modules/liveswap/HEAD'
    at Object.fs.openSync (fs.js:427:18)
    at Object.fs.writeFileSync (fs.js:966:15)
    at writeHEAD (/usr/local/lib/node_modules/liveswap/index.js:20:13)
    at module.exports (/usr/local/lib/node_modules/liveswap/index.js:42:3)
    at Object.<anonymous> (/usr/local/lib/node_modules/liveswap/bin/liveswap:117:22)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
```

By making the headpath configurable with `-H, --head` you can now run globally installed liveswap without using `sudo`.