ENH: Set default dialect to None to enable SQL prefixes
Alters the ```read_gbq``` function to leave ```dialect``` unset by default. This allows the user to specify the dialect using a [query prefix](https://cloud.google.com/bigquery/docs/reference/standard-sql/enabling-standard-sql#sql-prefix), while leaving the default behavior unchanged.
cc @parthea  can you add a test for this? (not sure if you need to modify any existing tests).

cc @parthea  I'm happy to add a test, but would appreciate some guidance into which cases it should cover. Here are the cases I'm thinking about:

- ```#standardsql``` hint works with no dialect passed
- ```#legacysql``` hint works with no dialect passed
- ```#standardsql``` works with ```standard``` passed as dialect
- ```#legacysql``` works with ```legacy``` passed as dialect  @cgrin I think just a test to exercise it would be great, e.g.

```
read_gbq(....) # default
read_gbq(..., dialect=....) # legacy
read_gbq(..., dialect=....) # standard
```

then compare those actually this should be migrated to https://github.com/pydata/pandas-gbq. pls open a PR there if desired.