Add support for DECIMAL type

[Alexandre Fiori]

Add support for fixed-precision type: https://prestodb.io/docs/current/language/types.html#fixed-precision

[Alexandre Fiori]

While looking at this I learned that Presto returns decimal types as literal strings. Perhaps this is why #11 and #12 wanted to implement this as strings.

I believe the right thing to do is to support decimals as float64 and sql.NullFloat64.

[David Phillips]

What does the PostgreSQL driver return?

[Alexandre Fiori]

github.com/lib/pq supports scanning decimals to string, float64, and their variants sql.NullString and sql.NullFloat64. Despite not being able to convert to int64 for obvious reasons, a CAST(NULL to DECIMAL(10,2)) can be scanned to sql.NullInt64.

I haven't tried github.com/go-sql-driver/mysql but I believe it should be consistent with pq.

That said, we should probably do the same here.

[Alexandre Fiori]

This is a simple fix. Added a suggestion in #11 (comment) which should provide the same behavior as pq.