Implement `@bors r+ squash`

[Kobzol]

It is natural to combine r+ and the squash operation, so it would be nice to add something like @bors r+ squash[=<commit message>], which would first start the squash operation and once it successfully finishes, it would approve the newly squashed commit and send a comment.

#692 should be implemented first.

[reddevilmidzy]

@rustbot claim