Plugin refinement

[magurotuna]

We already have a prototype of plugin, but it needs to be refined further to make it ready for use. This issue gives a TODO list of necessary refinements.

• making plugins writable in TypeScript, maybe powered by the internal mechanism of Deno CLI
• allowing to fetch remote modules, which also may be powered by Deno CLI
• switching from visitor.js to the Deno version of ast-view
  • can we make this function signature more type-safe? module: any cannot be changed?
• considering how to enable plugins via config file
  • it would be good to have some mechanism to "group" additional rules, like ESLint does
  • users would write like:

import NoConsole from "./no_console.ts";
import Eqeqeq from "./eqeqeq.ts";

DenoLint.register({
  name: "mySuperPlugin",
  rules: [
    NoConsole,
    Eqeqeq,
  ] 
});

• writing documentation on how to implement plugins
  • at least an actual implementation example of a plugin would be fine
  • complicated examples are not really necessary at the initial stage