drop index 'ifExists' and createIndex 'ifNotExists' options

[kevinand11]

I'm submitting a...

• [ ] Bug report
• [x] Feature request
• [ ] Question

Current behavior

Expected behavior

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

Environment

db-migrate version: X.Y.Z
plugins with versions: X.Y.Z
db-migrate driver with versions: 

Additional information:
- Node version: XX  
- Platform:  

Others: