Reading data-${platform}.sql in addition to schema-${platform}.sql, schema.sql, data.sql

It took me some time to realise why only `schema-{platform}.sql` file is loaded on startup, `data-{platform}.sql` is not. To be fair, [documentation][1] mentions this but I didn't focus on that part too much since I thought it was something obvious. Only after analysing the code I saw that it was not included in the scanned list.
I'm assuming there *was* a reason to leave it out but it feels like data script would be more likely to be platform-specific than schema. In my case `data-h2.sql` would only be loaded to seed the in-memory database for tests and development mode. Production database has real data and I don't want to touch it during application's startup so using `data.sql` is out of the question.

This PR adds `data-{platform}.sql` to the list of parsed SQL scripts, also updated the relevant part of documentation. I don't think it could do any harm, hope there won't be any reasons to reject this proposal. If there's a legitimate reason for not allowing this, I feel documentation should offer alternative way of doing this (looking at [the code][2], it would most likely be specifying value of `spring.datasource.schema` by hand?)

[1]: http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#howto-intialize-a-database-using-spring-jdbc
[2]: https://github.com/spring-projects/spring-boot/blob/50190a4de7dec55df462fb06c2d888176403d654/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/DataSourceAutoConfiguration.java#L97

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