Add support for MySQL

[felippepuhle]

As MySQL user, it would be interesting to have this feature too.

[sibelius]

TODO

• read mysql table definition
• add config to .graphqlrc
• modify type to work with mysql
• modify loader to work with mysql
• modify mutation to work with mysql
• modify connection to work with mysql

I think we should have a generic approach to support any database, maybe a plugin style design

templates generators should receive an array of fields with all information needed

[sibelius]

This could help https://github.com/rexxars/sql-to-graphql?

[lucasbento]

@sibelius: yeah.

We should start working on #59 asap.

[sibelius]

this could help us: https://github.com/gajus/database-types