try to load `cz-conventional-changelog` by default

Load it by default, without the need for config, which currently is

``` json
{
"config": {
"commitizen": {
"path": "./node_modules/cz-conventional-changelog"
}
}
}
```

which i believe it should be handled by the cli. it is so trival and basic stuff. and hance, it is "path", i think it would make more sense to be `preset` and try to load this preset automagically.

What i suggest? It should try to load from local (cwd) node_modules, then try to load from global.