On P, you get an "EDIT" option when you take a screenshot. That starts an editor for the image, and Google have included an app on the Pixel 2 builds called "Markup", which allows for cropping and simple editing:
This is a backport of that app, with only one modification, the addition of the "android.intent.action.SEND" intent filter. This means that on Oreo and below, Markup can be accessed after you take a screenshot by pressing "SHARE" and then selecting markup.
Usage (large GIF, view the gifv here):
Install it as a normal APK