Give user's eslint configuration precedence for coverage analysis

#### Support plan

* *is this issue currently blocking your project?* (yes/no): no
* *is this issue affecting a production system?* (yes/no): no

#### Context

* *node version*: N/A
* *module version*: latest
* *environment* (e.g. node, browser, native): node
* *used with* (e.g. hapi application, another framework, standalone, ...): whatever project
* *any other relevant information*:

#### What problem are you trying to solve?

@kanongil [suggested](https://github.com/hapijs/eslint-plugin/pull/27#issuecomment-830041513) that instead of relying on `babel-eslint` directly `lab` should look beforehand into the project's eslint configuration to check whether the user uses another parser. If that's the case then we'd use this parser for the coverage analysis otherwise if the user didn't declare any parser or has no eslint configuration we'd default to hapi eslint plugin value.

#### Do you have a new or modified API suggestion to solve the problem?

This doesn't need any change to `lab`'s API.