Add Postgresql support

[goldenbull]

Any plan for postgresql? I can try to write some code. The mysql implementation is a good reference.

[VShawn]

I have never used PostgreSQL before, so it would be great if you could help me with making and testing the PostgreSQL module.

[goldenbull]

尝试了一下，新增界面、连接pgsql都没问题了，目前主要的困难在于SQL语句的兼容性：

1. table name和field name：mysql和sqlite的SQL使用back tick，pgsql使用double quotes
2. 字符串：mysql和sqlite使用double quotes， pgsql使用single quotes

目前看起来所有的SQL都在 DapperDataBase.cs 中，我再看看dapper是否可以提供一致的支持

[goldenbull]

Writing SQL that works on PostgreSQL, MySQL and SQLite
Quoting differences between MySQL and PostgreSQL

[goldenbull]

PR created 😄