Support triggers [community contribution welcome]

Triggers can be supported by `gh-ost`. This is not on our immediate roadmap, and so it's an ideal opportunity for community contribution.

Some notes:
- as far as I can see, all sorts of triggers could be supported (`before|after`, `insert|delete|update`)
- throughout the migration process, nothing is really done; the `ghost` table should NOT have triggers.
- during the [cut-over phase](https://github.com/github/gh-ost/blob/8fbff6519f37cf1c8769af3520ec2fd12efeff1b/go/logic/migrator.go#L422-L439) we can copy+paste the triggers from original table to _ghost_ table
- Preferably the triggers are read at migration start, not at cut-over phase
- triggers can be applied on the _ghost_ table after the backlog is drained and the tables are in sync (after `this.waitForEventsUpToLock()`)
- triggers must be removed in any case of rollback, and in such case, before `gh-ost` resumes writes onto the _ghost_ tables (no problem if original table resumes writes)
- _Note_ that at this time `gh-ost` supports two types of cut-over, and both must be supported.

Anyone who is interested to jump on this please let me know beforehand. Thank you!