SQL service

[sebinside]

Close #223 by using knex with mysql as default client. Can be used with any other supported client, though.

Progress:

• Create service client
• Test service client
• Add example bundle
• Add some knex example code w/ explanations
• Remove mysql from dependecy list, update comments

[sebinside]

Still contains a reference to mysql in the package.json to make the service easy to get started with. Could be removed together with a change to the comment in the description in nodecg-io-sql/package.json:4.

[hlxid]

Please remove nodecg-io-sql/package-lock.json. It is created when using e.g. npm install sqlite3 inside the sql service. Add your dependencies manually to the corresponding package.json file and the run npm run bs in the repository root (this also needs to be done to update the root package-lock.jn)

If you will add a explanation to the docs on how to install support for other sql dialects please use npm install sqlite3 --no-save. This will create no lockfile because for when a user manually installs a dependency these are not really required anyway.

I don't really care whether we want to include mysql by default. On the one hand it makes it easier to get started, on the other we could make the installation smaller for people not using mysql. I'm fine with both, do want you think is better. We still can change it after merging this PR.