Automatically ref names of CTETables in DELETE and UPDATE statements

[huandu]

Per #176, The names of CTE tables created by CTETable can be automatically added to FROM clause in DELETE and UPDATE statement. The UPDATE statement in MySQL is an exception as MySQL uses different syntax.

See ExampleCTEBuilder_update and ExampleCTEBuilder_delete in ./cte_test.go for more details.

[coveralls]

coverage: 96.807% (-0.06%) from 96.87%
when pulling 96c9b25 on feature/cte-table-for-update-delete
into 134c901 on master.