Add support for LATERAL joins

[NikolayMetchev]

Postgres has added support for LATERAL joins:
https://www.postgresql.org/docs/9.4/queries-table-expressions.html

Can we please add support for those types of join to exposed?

[damien-roche]

@Tapac is this something you would accept PRs for?

I would really love support for this as would simplify a few of the queries I'm working on.

My plan would be to add a joinModifier to the JoinPart along with enum JoinModifier.LATERAL and then append the modifier here:

https://github.com/JetBrains/Exposed/blob/master/exposed-core/src/main/kotlin/org/jetbrains/exposed/sql/Table.kt#L215

I am, however, unsure of where best to pass the modifier.