Quality of life improvements to allow easier configuration

[kelloggm]

This change has two parts, both of which are motivated by some of my experiences trying this plugin out on an Android app with a checker that isn't distributed with the Checker Framework.

The first change is the addition of an easy way to add custom arguments via the plugin. This is very convenient when using stub files or other Checker Framework options, like -AstubDebug.

The other is instructions in the README about how to use the plugin with a third party checker.

I tested both locally.

[jaredsburrows]

Can you update this so we can merge it?