ConfigReader support for Java BigDecimal and BigInteger

[vlovgr]

Ciris currently only have ConfigReader instances for scala.math.BigDecimal and scala.math.BigInt, but not the underlying Java java.math.BigDecimal and java.math.BigInteger. We could use the existing Scala support and simply use underlying to get the Java versions.

@ BigInt(123).underlying
res0: java.math.BigInteger = 123

@ BigDecimal(123).underlying
res1: java.math.BigDecimal = 123

[vlovgr]

Fixed by #57.