Allow merge or rebase in mergeStrategy

**Is your feature request related to a problem? Please describe.**

When setting your `mergeStrategy` to ` toReleaseBranch`, Ship.js creates a merge commit in the `develop` branch (merges `master` back). This creates a messy history, and doesn't make both branches exactly equal (same head, same tag) at a given moment, which is what one may want.

**Describe the solution you'd like**

It would be nice to be able to specify whether we want to `merge` or `rebase`:

```js
module.exports = {
mergeStrategy: {
toReleaseBranch: {
develop: 'master',
},
mode: 'rebase', // or 'merge'
},
};
```

Start by locating the JavaScript configuration and release-flow entry points that handle mergeStrategy and toReleaseBranch, then trace how develop and master are synchronized. Done means the configuration can select merge or rebase behavior and the resulting branch history matches the selected mode.

Written by the indexing model from the issue text.