Add a linter script

[sharmaeklavya2]

We should have a script which lints all markdown files, like checking for trailing whitespace, trailing newlines, no newline at end of file, etc.

It would be cool if we could use a library already built for this purpose (since it would be much more feature-filled). But a solution written from scratch is also acceptable.

After that's done, we can maybe add a Travis CI check to automatically lint all pushes and pull requests.