proposal: coerceTypes from strings only

**What version of Ajv you are you using?**
6.10.0

**What problem do you want to solve?**
I want to convert values from queryStrings and url paramters into to their proper type. CoerceTypes `true` and `"array"` seem too eager to make changes I don't want such as converting `false` to `0` and vice versa.

**What do you think is the correct solution to problem?**
Add a new option for `coerceTypes`, maybe `"string"` or `"fromString"`.

**Will you be able to implement it?**
Probably not soon.