Add support for variable substitution for `minNumRows` in `rowCount` check

[phpisciuneri]

We should be able to support the following use case or something analogous to it:

---
numKeyCols: 2
numErrorsToReport: 1
detailedErrors: true

vars:
  - name: NUM_ROWS
    value: 1000
  
tables:
  - db: census_income
    table: adult
    checks:
      - type: rowCount
        minNumRows: $NUM_ROWS

Currently, trying the above configuration yields a fairly non-descriptive DecodingFailure

21/01/15 12:24:03 ERROR Main$: Failed to parse config file 'issue.yaml, {}
DecodingFailure(Attempt to decode value on failed cursor, List(DownField(parquetFile), DownArray, DownField(tables)))

It is noted in the documentation that this is not currently supported.