Allow merge or rebase in mergeStrategy

[sarahdayan]

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:

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