Allow multiple files and directories to be linted

[anderseknert]

The first iteration of the executable currently works with a single file only. We should improve this and allow any number of files and directories to be scanned. Running the entire set of rules against the AST of each individual Rego file is likely going to be quite expensive, and later we'll want to do some pre-processing of the files to create new datastructures more easily traversed (#13), but we can start with a naive implementation and just create an array of all the files (or rather, the AST representation of each), and scan them one-by-one (possibly in paralllell).

[anderseknert]

Passing a directory, or multiple files, to the lint command now works. We might still want to create an issue later for "merging" all the modules into a single big one, but let's create a separate issue for that if need be.