Adds fixes for fetch URL PR

[tomnomnom]

A great start!

Here's a few suggestions/tweaks to your PR (so this is a PR for your PR - yo dawg).

1. Updated the help and readme to reflect the changes
2. Added a new exit code so that failing to fetch a URL is distinct from failing to read a file
3. Renames url_tests.go to url_test.go; that's what go test expects, your tests weren't actually being run ;)
4. Refactored the 4 test methods into a single table-driven test.

Table-driven tests are super useful when you want to test a bunch of inputs to a single function, and massively reduce the scope for typo or copy+paste type errors.

You definitely did the bulk of the work here though, so thanks again!

Also, nice one for using 'gron' as a verb; I think I might using 'gronning' as the term for transforming an object to discrete assignments from now on :-D

If you're happy with my changes then merge them and I'll accept your original PR.

Thanks!

[iamthemovie]

Thanks Tom.

Ahh you know what happened with those tests? I had my editor set to use spaces not tabs and gofmt complained (as it would) so I ran gofmt and re-outputted the file to the wrong file name facepalm.

"gron" just rolls off the tongue really!

I'll merge this in now.