Allow purely random assignment

[tir38]

Feature Request:
We'd like to bypass most of the algorithm and allow for a purely random assignment.

Use case:
Our code base has been heavily modified by one person. This person usually gets assigned most PRs for review. However we want new employees to get a good understanding of the project so we want them to get assigned PR review for code they have not yet contributed to.

[imsky]

sure, seems like a straightforward feature to implement